Mac and Dennis have known each other since high school. They became friends despite coming from entirely different social circles - according to Dennis' recollections of high school, Mac was a drug dealer with no friends while Dennis was immensely popular ("Underage Drinking: A National Concern"). However, we later learn that Dennis was not as popular as he claims to have been, and spent all of high school getting high under the bleachers with Mac and Charlie ("The High School Reunion, Part 2: The Gang's Revenge"). Even though they were friends, Mac slept with Dennis' prom date and covered it up by telling Dennis that Tim Murphy did instead. Dennis doesn't find out the truth until years later ("The High School Reunion, Part 2: The Gang's Revenge"). By season four, Dennis and Mac have been living together for approximately ten years, meaning that they moved in together sometime in 1998 ("Mac and Charlie Die (Part 1)"). At some point after Dennis's graduation from the University of Pennsylvania, Dennis and Mac bought Paddy's Pub along with Charlie. The two of them convinced Charlie to trade his shares of the bar to them for Garbage Pail Kids card and half a sandwich, and are thus currently the only shareholders ("The Gang Sells Out").
Initially, Mac and Dennis are shown to have a rather antagonistic relationship. Even though they are best friends, this is consistent through all the seasons of the show, but especially in the first season. Mac tries to put Dennis in harm's way by getting him blackout drunk as part of a scheme to end Dennis's insistence upon running a gay bar ("The Gang Gets Racist"). He and Dennis also compete for a woman's affections when she comes to pick up her dead grandfather from their bar, with Dennis eventually sleeping with the woman and infuriating Mac enough to tell Dennis that the Reynolds' own grandfather was a Nazi ("The Gang Finds A Dead Guy"). Their friendship is more positively portrayed when, along with Charlie, they buy a gun together and become obsessed with it ("Gun Fever").
Mac sleeps with Dennis's mom, an experience which he describes as "special and magical." Dennis punches him in the face for it and attempts to get revenge on him by sleeping with Mac's mom, but the two of them are later seen drinking together at Paddy's and appear to have reconciled ("Mac Bangs Dennis' Mom"). When Dennis and Mac coerce Charlie into joining an underground fight club and Charlie bails on them at the last minute, Mac steps in to fight in his place, and later discovers that Dennis had so little faith in his ability that he bet all his money on the other fighter to win ("Hundred Dollar Baby"). Their competitive streak continues while coaching opposing basketball teams as part of required community service ("The Gang Gives Back"), and also when Mac runs a smear campaign against Dennis when he runs for comptroller ("The Gang Runs For Office"). Together, they decide to make Paddy's Pub an "anything-goes bar," until the invasive presence of the incestuous McPoyles and Frank's gambling friends persuades them otherwise ("Charlie Goes America All Over Everybody's Ass").
This season contains some of the earliest hints of homoeroticism that are prevalent throughout Mac and Dennis's relationship. When Dennis's mom dies, Mac, Dennis, and Charlie throw a party in Dennis's newly inherited mansion. To do this, they try to recruit partygoers with posters that end up shaped like penises, and Mac and Dennis in particular are fixated on finding attractive men to attend their party ("Dennis and Dee's Mom Is Dead"). When the gang is held hostage by the McPoyles, Mac abruptly confesses to Dennis that he loves him. The confession goes unreciprocated and neither of them make any mention of it again ("The Gang Gets Held Hostage").
When Mac, Charlie, and Frank decide to start a band, Mac enlists Dennis to replace Charlie after hearing Charlie perform a disturbing song about rape. However, he changes his mind after witnessing Dennis's spandex-clad, glam-rock-inspired performance style, and kicks Dennis out of the band as well ("Sweet Dee's Dating a Retarded Person"). Dennis, along with the rest of the gang, briefly suspects Mac of leading a double life as a serial killer, but appears to have no moral objection to this, and even gets excited when he and Dee try to get into the serial killer mindset themselves ("Mac Is A Serial Killer"). After witnessing how Frank has brainwashed Dennis into prostitution and endangered them all by almost sleeping with a mob boss's wife, Mac slaps Dennis around until Dennis starts thinking for himself again ("The Gang Gets Whacked (Part 2)"). When the bar holds a dance contest, Mac challenges Dennis to a dance competition because he says he is tired of Dennis not appreciating him, but loses after performing a series of dance moves that includes miming the act of fellatio on Dennis ("The Gang Dances Their Asses Off").
Mac and Dennis team up to hunt Rickety Cricket across the city for fun. The two make repeated references to teabagging Cricket, and before the hunt is over they perform variations of this act upon one another. Frank expresses how he "doesn't understand [their] generation" based on the fact that Mac and Dennis keep touching each other in ways that he deems inappropriate ("Mac and Dennis: Manhunters"). The gang holds a modeling contest and Dennis wishes to be one of the hopefuls, but is discouraged by Mac, who insists that while Dennis used to be "a stallion," he is no longer in his physical prime, and is no competition for the hordes of "beefcakes" that he is up against ("America's Next Top Paddy's Billboard Model Contest").\\
This season begins to hint at the strength of Mac and Dennis's codependence, particularly in the episode "Mac's Banging the Waitress". When Dennis overhears Mac and Charlie describe themselves as best friends, he quickly grows jealous and tries to turn Mac and Charlie against each other, even convincing a drunk Charlie to get in bed with him. Mac admits that he and Dennis have a bond much stronger than his bond with Charlie, describing them as "blood brothers." Privately, Dennis and Charlie agree that Mac is probably gay, speculating that he's in love with both of them and that his Project Badass videos are a way to get their attention.
When Mac and Charlie fake their deaths to hide from Mac's dad, Dennis realizes they're still alive almost immediately; however, angry that his friends didn't include him in their suicide pact, he taunts Mac by throwing out all his possessions and getting a brief replacement roommate ("Mac and Charlie Die (Part 1)"). After Mac's dad flees the country and Dennis forgives Mac and Charlie, Mac moves back into their apartment ("Mac and Charlie Die (Part 2)"). In order to add truth to Dennis's largely fabricated memoir, Mac gives Dennis nailgun stigmatas and, intending to drop him off in front of an asylum, accidentally leaves him for dead at an abandoned warehouse ("Dennis Reynolds: An Erotic Life"). When Charlie comes to the gang with his musical, The Nightman Cometh, Mac and Dennis switch the roles Charlie intended for them, with Mac playing the Nightman and Dennis playing the Dayman. They spend their rehearsal arguing over how to make the "rape scene" in the show as tasteful as possible, deciding to cover themselves with a blanket while Mac dry-humps Dennis. Although neither of them find anything homoerotic about this in the rehearsal, Dennis calls Mac out on getting an erection during the actual performance ("The Nightman Cometh").
By this point in the show, Mac and Dennis's friendship and partnership is the fulcrum of many of the gang's schemes. They pretend to be realtors in order to sell Frank's house, creating personas "Hugh Honey" and "Vic Vinegar" for themselves, and develop these characters into a rich homosexual couple with explicit detail about who is the top and who is the bottom ("The Gang Exploits The Mortgage Crisis"). The two of them discover their inability to run the bar by themselves when they fire the rest of the gang and end up losing an inordinate amount of money ("The Great Recession"). In a rare moment of kindness, they work together to get Charlie a date after they find out that the Waitress is engaged, but their ever-present need to one-up each other is part of what ultimately foils the plan ("The Waitress Is Getting Married"). Recounting the events of the day of the 2008 World Series in court, Dennis mocks Mac for his crush on Chase Utley, and the two engage in a pushup competition whose winner is ambiguous ("The World Series Defense"). Together with Charlie they create short-lived wrestling team Birds of War ("The Gang Wrestles For The Troops"). After hearing Charlie's pitch for kitten mittens, Mac and Dennis conceptualize a number of their own ideas for bar merchandise, including dick towels, erotic pens, and alcohol-filled shotguns ("Paddy's Pub: Home of the Original Kitten Mittens").
The episode "Mac and Dennis Break Up" is entirely focused on their attachment to one another, and what happens when the two of them are separated. We learn that Mac and Dennis never go more than an hour without checking in, that they have a movie night every Tuesday (during which they typically watch Predator), and that they think of themselves as a "dynamic duo." Dee calls them out on acting like an "old married couple" and describes their relationship as "pathetic," which unsettles Dennis enough that he convinces Mac to move out. However, they spend the entire episode obsessing over each other and trying to use their friends as replacements, eventually irritating Dee enough into reuiniting them by secretly setting them up on a date and reassuring them that "it's perfectly natural for two grown men to need each other this badly."
When Dennis outlines his sick "system" for seducing women to rest of the gang, we learn that Mac plays a pivotal role in it; later, Dennis discovers that Mac has been sleeping with all the women Dennis sleeps with once Dennis decides he's done with them, something Mac describes as "swimming in your wake," and that Mac is not just his wingman but also his dedicated follower ("The D.E.N.N.I.S. System"). When Mac and Charlie attempt to pitch a movie idea to M. Night Shyamalan and run their ideas by Dennis, Dennis acts disinterested, but eventually reveals that they had his rapt attention and he was secretly developing a full screenplay out of their story the entire time ("Mac and Charlie Write a Movie").
The season opens with Mac having a crisis, his transmisogynist attitudes leading him to believe that his ex-girlfriend Carmen, a trans woman, is "gay married" to her husband, which Mac's Catholicism denotes is a sin. During Mac's mission to eradicate gay marriage, Dennis is inspired to marry his high school sweetheart Maureen Ponderosa, and effectively uses her to replace Mac, once again kicking Mac out of their apartment and taking Mac off their cell phone's family plan ("Mac Fights Gay Marriage"). Mac becomes convinced, however briefly, that "anyone should be able to get married," possibly because being married to Dennis would have legally protected him from being thrown out and cut off by Dennis as quickly as he was. Dennis and Maureen's relationship quickly unravels, and when Dennis comes home with Mac after a night of going out drinking, Maureen tells Dennis, "I want you and your boy toy out of this house." Mac eventually convinces Dennis to go through with divorcing Maureen, which leaves Dennis saddled with Maureen's enormous debt ("Dennis Gets Divorced").
When Mac realizes that Dennis intends to use their newly acquired boat to rape women, he becomes extremely uncomfortable, and tries to convince Dennis that his methods of "seduction" via what Dennis calls "the implication" are actually violent and dangerous; eventually Dennis convinces Mac to follow his lead, and the two of them lure a pair women back to their boat only to find that Dee, Charlie and Frank have burned it down ("The Gang Buys A Boat"). Dennis establishes his belief that he and Dee are higher-class than Mac and Charlie, something Mac is insecure about ("Mac and Charlie: White Trash"). Dennis briefly fears that he was complicit in killing Mac's dog, but Mac doesn't worry at all and the dog is revealed to be alive and well ("Mac's Mom Burns Her House Down"). Dennis, along with Charlie and Frank, suspects Mac of having drunkenly had sex with with Dee at the Halloween party, until Mac recounts that he had sex with Margaret McPoyle instead, and the McPoyle brothers reveal that Dennis is the most likely one to have slept with Dee that night ("Who Got Dee Pregnant?").
Mac and Dennis both try to reenlist Schmitty into the gang, even kicking Charlie out after Charlie gives them the ultimatum "it's me or him," but Schmitty is uncomfortable with how codependent they are, commenting on how it's weird that Dennis orders food for everyone just because he claims to have "the most refined palate," how creepy their rituals for re-initiation are, and how they don't have to take their friendship so seriously, which deeply rattles and angers Mac and Dennis enough to get rid of Schmitty and come crawling back to Charlie ("The Gang Gets A New Member").
Their mutual love for eighties movies, as well as their different but equally disturbing views on the use of blackface, are showcased in their homemade movie Lethal Weapon 5, in which Frank and Charlie are supporting actors ("Dee Reynolds: Shaping America's Youth"). When Dennis and Charlie pretend to be Mac and Frank at the animal rights charity event in Atlantic City, Dennis gets to play catch with Chase Utley, something Mac has always wanted to do, and taunts Mac by drunkenly sending him a picture of himself and Utley ("The Gang Gets Stranded In The Woods"). When the gang thinks that Dee is keeping her baby and worry that they'll be stuck with all the responsibility of caring for it, Dennis actually gets excited about raising the baby with Mac and Charlie in a "Three Men and a Baby-type-situation" ("Dee Gives Birth").
Season EightIn the second episode of this season, while Dennis, Mac and Charlie are discussing whether they should begin selling gas door-to-door again, Mac goes in to kiss Dennis. Though he is unsuccessful and strolls off as if nothing has happened, it does not go unnoticed by both Dennis and Charlie ("The Gang Recycles Their Trash").
Dennis gives an impromptu speech about Mac in front of everyone at Gugino's, calling Mac his "hero" and "the wing beneath [his] wings." ("The Gang Dines Out")
While trying to catch a rat, Dennis argues that "seducing" the vermin is the best way to corner it, then turns on the song "You're The Inspiration" by Chicago, and he and Mac sway towards each other, with Mac eventually leaning in towards Dennis before Dee interrupts. ("Flowers For Charlie")
Mac and Dennis move to the suburbs and attempt to live there for a month. This turns sour when they realize they are meant for city life– Dennis resents the neighbors and the long commute to work, while Mac resents staying home and cooking and cleaning for Dennis. In an emotional outburst over dinner, Mac tells Dennis, "Everything I do, I do for you!" ("Mac and Dennis Move To The Suburbs")
Mac approaches Frank about Dennis, saying, "Do you think Dennis hates me? I just want him to think I'm cool," and "I put so much of my heart and soul into our relationship." ("Being Frank")
- It is revealed by Artemis that Dennis listens to all Mac's phone calls ("Who Pooped the Bed?").
- Mac and Dennis have Tuesday "movie night" every week. (Mac and Dennis Break Up)
- Mac insists that he peel all of Dennis' apples before eating them, because the skins are "loaded with toxins". (Mac and Dennis Break Up)
- Mac has trouble finding the right word to express his ideas sometimes, at which point Dennis often suggests the perfect word for him. (Mac and Dennis Break Up)
- Dennis orders salad at a restaurant for Mac even though Mac hates salad. Dennis has the more refined palette. (The Gang Gets a New Member)
- Mac and Dennis have had four episodes named after them:
- Mac and Dennis have a monthly Dinner at Guiginos. ( "The Gang Dines Out" )
- Mac and Dennis lived together for 15 years before their apartment burned down and they had to move in with Dee. They briefly tried to live in the suburbs together but both of them went insane from living there and moved back to Philadelphia. They currently sleep in a California King Size Bed with Dee and an old man who may or may not also be living full time in Dee's apartment.