Keith John Moon was born on 23 August 1946 at Central Middlesex Hospital, Park Royal, London to Alfred and Kathleen Moon. He lived in Wembley as a boy, was extremely hyperactive, and had a restless imagination. Alfred's nickname for his son was "Nobby." As a youth, one thing that could hold his attention was music. A report from his secondary modern school is not encouraging – his art teacher, for example, commented: 'Retarded artistically. Idiotic in other respects. Teacher Aaron Sofocleous praised his music skills and encouraged his chaotic style, even if one school report noted "He has great ability, but must guard against a tendency to show off." Moon failed his eleven plus exam and left school in 1961.
On 17 March 1966, Moon married his pregnant girlfriend Kim Kerrigan in secrecy. Their daughter Amanda was born four months later, on 12 July. In 1973, Kerrigan left Moon, taking Amanda with her. They divorced in 1975.

At 17, Moon joined The Who (in April 1964), a replacement for their original drummer Doug Sandom. Sandom had left the band less than a month earlier and the remaining members hired a session drummer to fulfill a run of shows that they had already agreed to play. Keith Moon was at one of these shows. Peter Townshend later described him as looking like a "ginger man" with his hair dyed ginger and wearing ginger-coloured clothes. As stated in Amazing Journey: The Story of The Who, Keith looked up to Roger Daltrey during the show and said "I hear you're looking for a drummer. Well, I'm much better than the one you've got."The band knew that they needed Moon after seeing him practically smash the drum kit to pieces.

Moon was highly destructive. He laid waste to hotel rooms, the homes of friends, and even his own home, often throwing furniture out of high windows and destroying the plumbing with fireworks. He frequently flushed powerful fireworks down the toilet and was known to detonate toilets for amusement. The acts though often fueled by drugs and alcohol, were also his way of expressing his eccentricity, as well as the undeniable joy he got from shocking the public. "He would do anything if he knew that there were enough people around who didn't want him to do it" -Dougal Butler (Full Moon)