Luther McDonald is Mac's father and a convicted felon.
Luther is tall, has numerous tattoos, has piercing blue eyes and mentions that he rarely blinks, giving him a generally intimidating appearance. His hair is always slicked back and he has multiple teardrop tattoos, indicating that he has killed people before.
Luther was born in 1956.
When Mac and Charlie first visited Luther in prison in an attempt to bond with him, Luther tries to convince them to smuggle heroin into the prison via their rectums, as a "father-son type of thing". ("Dennis and Dee Get a New Dad")
While out on parole, Luther convinces Mac to help him "take care of some people". The warm and gentle Bonnie Kelly is attracted to Luther's aloof behavior and criminal past, and Luther openly flirts with her during a family dinner which Mrs. Mac and Frank (who was in a relationship with Mrs. Kelly at the time) were also attending, much to Mac's dismay and Charlie’s delight, as Luther could steal Mrs. Kelly away from Frank, who was stolen away from Charlie by Mrs. Kelly.
Later, both Charlie and Mac come to believe that Luther was killing the people he said he was taking care of and end up getting him arrested after finding their own names on the list of people he planned to "take care of". However, they later discover that he was in fact visiting people who he had wronged in his criminal past, and was issuing sincere apologies to them. Despite the fact that he wasn't actually hurting anyone, 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 as a surprise, knowing that Mac had always wanted to have a catch and play baseball with him.
Luther blamed Mac and Charlie for his going back to prison, as they were the ones who called the police and ultimately got him arrested just days after being granted probation. ("Dennis Looks Like a Registered Sex Offender")
Mac and Charlie panic upon hearing that Luther is scheduled for a parole hearing, thinking that Luther will try to kill them as soon as he's out of jail. They plan a scripted court statement to prevent Luther from being granted parole, but unbeknownst to them, Luther had recorded the absurd conversation between the two of them in which they first discussed framing him by claiming that he had threatened to anally rape them both if he was released. The court easily sees through Mac and Charlie's phony testimony and grants Luther parole.
Terrified, Mac and Charlie concoct an increasingly convoluted plot to fake their own deaths, which quickly escalates to the point that they steal Dee's car, intentionally crash it into a brick wall, and then set a hand grenade off inside of it, which does much less damage than they expect.
However, Luther never planned on hurting them, despite being understandably furious with them. Too angry to confront them, he writes them a letter in which he explains that he still loves Mac, but can't stand to be around him or Charlie for the time being, and to not contact him under any circumstances. He then moves to a beach in Tijuana to enjoy himself far away from the two of them. ("Mac and Charlie Die (Part 1)") ("Mac and Charlie Die (Part 2)")
In a home video tape, it's revealed that Luther, Mrs. Macdonald, and Mac used to break into houses on Christmas to steal Christmas presents, apparently convincing Mac that this was a normal Christmas tradition in which all Philadelphia families participated. ("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, who had evidently been staying at Charlie's mother's house for several months until then, and who had had at least one threesome with Luther and Bonnie Kelly while staying there. Mrs. Kelly describes this in painstaking detail when Mac and Charlie confront her and Mrs. Mac about the murder, explaining that the initial threesome eventually turned into Luther penetrating Eduardo, which Mac tries to excuse as his father "dominating" another man. This fact does lead to the charges against Luther being dropped, but when Mac and Charlie go to tell him the good news, Luther is furious and explains that now the other inmates think he had his son go to the police and have Eduardo arrested for murder, and that they'll now try to kill him in prison for being a rat. ("Mac Kills His Dad")
Luther appears in two nightmares that Mac has, due to being stressed out and suffering what appears to be PTSD resulting from a VR video game that he and Frank play. In the first dream, a man who Mac shoots in the game turns out to be Luther, who tells Mac that he's killed him, and dies just as he is about to tell Mac he loves him. In the second dream, Luther tries to smother Mac with a pillow. ("PTSDee")
Mac attempts to come out as gay to Luther. Luther initially believes Mac is going to be a father, but Mac later officially comes out with a five-minute modern dance sequence. Luther leaves in the middle of this dance. ("Mac Finds His Pride")
Luther is a drug dealer and very intimidating man. Due to being in prison he rarely saw Mac. Despite threatening to kill Charlie and Mac for turning him in, he sometimes seems to care for his son. Luther broke the conditions of his 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")
- Luther: I wasn't gonna kill you. But I'm going to now. Yeah. As soon as I get out, I'm gonna kill ya. See ya son. See ya real soon." ("Dennis Looks Like a Registered Sex Offender")
- Mac: “You didn’t do it, right dad?”
- Luther: “………Nooooooooooo.”
- Mac: “There’s no way you could have done this. You have a bad shoulder, and that’s why you could never play catch with me, remember?”
- Luther: “……….Riiiiiiight.” ("Mac Kills His 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 Six: "A Very Sunny Christmas"
- Season Ten: "Mac Kills His Dad"
- Season Eleven: "The Gang Goes to Hell: Part Two" (mentioned only)
- Season Twelve: "PTSDee" (dream)
- Season 13: "Mac Finds His Pride"
- It’s Always Sunny: Gang Goes Mobile
- 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 was initially unknown if Luther was still alive or not: he said that he will be killed because Eduardo Sanchez' gang now believes he ratted Eduardo out. In "The Gang Goes to Hell: Part Two" we hear that Dennis has been hiding the letters Luther sends to Mac, confirming he is still alive and in prison (evidently asking Mac to smuggle more things in his ass). In Mac Finds His Pride, it is shown that Luther is still alive.
- Luther appears to be bisexual, enjoying sex with Eduardo Sanchez to the point that Bonnie Kelly felt excluded. Mac denies this, believing he was "asserting his dominance." It is possible Luther is insecure about his sexuality in the same way Mac used to be.
- Despite this, when Mac comes out to Luther, Luther walks out in the middle of Mac's dance performance.
- In "Mac Kills His Dad", the news report mentions that Luther is 59 years old, and since this episode takes place in the year 2015, that would make Luther's birth year 1956, which makes sense since it's the same birth year as the actor who plays him.
- A recurring theme is Mac has never heard Luther openly say that he loves him, with the only minor exception being when he left Mac a letter claiming that he had forgiven him and stilled loved him.