Neil James Innes was born in Danbury, Essex, and studied at the Norwich School of Fine Art, from which he was thrown out around 1963, allegedly for "spending all day playing music, instead of making things".

In the period 1962 to 1965, Innes and several other art school students started a band which was originally named The Bonzo Dog Dada Band after their interest in the art movement Dada, but which was soon renamed the Bonzo Dog Doo-Dah Band (Often shortened to The Bonzo Dog Band). Innes, with Vivian Stanshall, wrote most of the band's songs, including "I'm the Urban Spaceman", their sole hit, (produced by Paul McCartney under the pseudonym Apollo C. Vermouth) and "Death Cab for Cutie" (which inspired an American musical group of the same name), which was featured in the Beatles' film Magical Mystery Tour.

Underneath the permissive, 1960s drug-influenced culture was a clear affection for a politically incorrect, earlier era of Empire and Gilbert and Sullivan word-play: "tigers don't go out on rainy nights, they've no need to 'whet' their appetites!"

In the 1970s, Innes joined with Eric Idle, of the Monty Python team, to create the television comedy series Rutland Weekend Television. This show spawned The Rutles (the "prefab four"), a Beatles parody band, in which Innes played the character of Ron Nasty, who was loosely based on John Lennon. Innes played Nasty in All You Need Is Cash.

The introduction and first line of the song How sweet to be an idiot was copied by Oasis in their song Whatever. "How sweet to be whatever I ...". Noel Gallagher, being a fan of the Beatles, was acquainted with Innes' work.

Innes also contributed to the Pythons' final BBC TV series in 1974 - he wrote a squib of a song called "George III" (sung by a pastiche black American girl group) which appears in the episode "The Golden Age Of Ballooning", he wrote the song "Where Does A Dream Begin?" (included in the episode "Anything Goes: The Light Entertainment War") and he co-wrote the "Most Awful Family In Britain" sketch in the last episode, "Party Political Broadcast". He is one of only two non-Pythons to ever be credited writers for the TV series, the other one being Douglas Adams (who co-wrote another sketch in "Party Political Broadcast," in which a patient profusely bleeding from the stomach is made to sign numerous senseless forms before being treated).

Innes wrote the songs for Monty Python and the Holy Grail, and appeared in the film as a head-bashing monk and the leader of Sir Robin's minstrels. He also had a small role in Terry Gilliam's Jabberwocky, and appeared with the Pythons at their legendary Hollywood Bowl concert. Because of these long-standing connections, Innes is often referred to as "the Seventh Python".

On BBC television, he performed songs and sketches in The Innes Book of Records, punning on the Guinness Book of Records. The series has not been repeated.

During the 1980s, Innes found a new, younger audience, when he played the role of the Wizard in the children's television series Puddle Lane.

He also voiced the 1980s Children's cartoon adventures of The Raggy Dolls, a motley collection of "rejects" from a toy factory. The 65 episodes for Yorkshire television included the characters Sad Sack, Hi-Fi, Lucy, Dotty, Back-to-Front and Princess.

Innes can occasionally be heard, (often as the butt of jokes) standing in as the pianist for the BBC Radio 4 panel game I'm Sorry I Haven't A Clue.