Luther McDonald is Mac's father and a convicted felon.
Luther is tall, has numerous tattoos, and mentions that he rarely blinks, giving him a generally intimidating appearance.
While out on parole, Luther convinced Mac to help him "take care of some people". The warm and gentle Bonnie Kelly was attracted to Luther's aloof behavior and criminal past.
Later both Charlie and Mac believed that Luther was killing the people he said he was taking care of and ended up getting him arrested after finding their names on the list of people he is gonna "taking care of". However, they later discovered that he was in fact making up to the people on the list who he believed he had wronged in his criminal past, but he was arrested anyway when the police discovered he was planning to violate his parole, by leaving the state to take Mac and Charlie on a trip to the Baseball Hall of Fame.
Luther blamed Mac and Charlie for his going back to prison, as they were the ones who originally called the police, and said that he would kill them. ("Dennis Looks Like a Registered Sex Offender")
Charlie and Mac tried and failed to sabotage Luther's parole. Then they both faked their own deaths to hide from him; however it turned out that Luther has forgiven them. He couldn't bear to see them though, so he moved to a beach in Tijuana. ("Mac and Charlie Die (Part 1)") ("Mac and Charlie Die (Part 2)")
In a home video tape, it's revealed that Luther, Mrs. Mac, and Mac used to break into houses on Christmas to steal Christmas presents. ("A Very Sunny Christmas").
Luther is charged with the murder (via decapitation with a stop sign) of a state's witness, and faces the death penalty. He refuses Mac and Charlie's offer to help free him, which they pursue anyway. They find out that the murder was actually carried out by a man named Eduardo Sanchez (along with some pretty uncomfortable details of a threesome Luther and Eduardo had with Charlie's mom.) This fact does lead to the charges against Luther being dropped, but Luther tells Mac and Charlie that now Eduardo's gang thinks he ratted him out, and so he'll probably end up getting killed anyway. ("Mac Kills His Dad")
Luther is a drug dealer, and a very intimidating man. Due to being in prison, he rarely saw Mac, but despite threatening to kill him and Charlie for turning him in, he seems to care for his son, planning on breaking parole to take them on a trip to the National Baseball Hall of Fame in Cooperstown. The second time he was out on parole, rather than kill them, Luther left for Tijuana.
- Luther: What do you guys know about smuggling heroin... through your anus? ("Dennis and Dee Get a New Dad")
- Luther: Listen to me. Do not come back here without anything up those ASSES!!!" ("Dennis and Dee Get a New Dad")
- Season Two: "Dennis and Dee Get a New Dad"
- Season Three: "Dennis Looks Like a Registered Sex Offender"
- Season Four: "Mac and Charlie Die (Part 1)," "Mac and Charlie Die (Part 2)"
- Season 6: "A Very Sunny Christmas"
- Season Ten: "Mac Kills His Dad"
- In his first episode, Luther was credited as "Bill".
- Luther's name is listed on parole papers as "Luther Mac". ("Mac and Charlie Die (Part 1)")
- Luther was held at Eastern State Correctional Facility for dealing meth. ("Dennis Looks Like a Registered Sex Offender")
- We finally hear him referred to by his full name, Luther McDonald, in the episode "Mac Kills His Dad."
- It's unknown if Luther is still alive or not: he says that he will be killed because Eduardo Sanchez' gang now believes he ratted Eduardo out, but we don't have an actual confirmation that he was indeed killed.