Growing up, Mackenzie was always very small for his age. During his teens, his parents were so concerned about it that they took him to a doctor who prescribed a growth hormone. The hormone seemingly worked, as Mackenzie shot up to a height of 5 feet, 9 inches.
Mackenzie is a committed environmentalist, going as far as to buy himself five acres of woodland in Essex.
Mackenzie broke a rib whilst filming a scene for the movie Three And Out. He was forced to work through the rest of the movie - which included his first sex scene - with the aid of painkillers.
Mackenzie has bred Mediterranean Spur-thighed tortoises since he inherited two tortoises, Joshua and Josephine, as a child.
Mackenzie played the role of 'Dutch Kid' in the 1998 movie, Still Crazy.
Mackenzie has two children; a son named Jude and a daughter, Scout.
He played Billy Bibbit in the 2004 London West End production of the stage play of One Flew Over the Cuckoo's Nest.
He is 5'9" (1.75 m) tall.
He lives in Peter Sellers's old semi-detached house in Muswell Hill, North London.
Before being cast as Gareth in "The Office" (2001) he used to work at Pizza Hut, in a chicken factory and at hospitals. He also did stand-up comedy for a decade.
(About his similarity to his character Gareth from "The Office")
Mackenzie: There are probably more resemblances than I'd like to admit. He's very pedantic. I don't think I'm pedantic in everyday life, but he loves trivia and facts, and that's what I'm into as well. And survival stuff; I'm quite into learning about that. I just have a tendency to get very nerdy about any hobbies I pick up.