Idle was born in South Shields, County Durham (now Tyne and Wear) in Harton Village, the son of Nora Barron (Sanderson) and Ernest Idle. His father had served in the Royal Air Force and survived World War II, only to be killed in a hitch-hiking accident on Christmas Eve of the first Christmas after the war . His mother had difficulty coping with a full-time job and raising a child, so when he was seven, she enrolled him into the Royal Wolverhampton School as a boarder.

Idle starred in the children's television comedy series Do Not Adjust Your Set opposite his future Python alumnus Terry Jones and Michael Palin (who were both former University of Oxford students). Terry Gilliam provided animations for the show. Other members of the cast were British comedic actors David Jason and Denise Coffey.

Idle wrote for Monty Python mostly by himself, at his own pace, although he sometimes found it difficult in having to present material to the others and make it seem funny without the back-up support of a partner. Cleese admitted that this was slightly unfair – when the team voted on which sketches should appear in a show, “he only got one vote” - but says that Idle was an independent person and worked best on his own. Idle himself admitted this was sometimes difficult: “You had to convince five others. And they were not the most un-egotistical of writers, either."

After Monty Python ceased to be a regularly active ensemble in the mid-1970s, all six members pursued solo projects. Idle's first solo work was his own BBC Radio One show, Radio Five (pre-dating the real Radio Five station by 18 years). This ran for two seasons from 1973 to 1974 and involved Idle performing sketches and links to records, with himself playing nearly all the multi-tracked parts.

He received good critical notices appearing in projects written and directed by others - such as Terry Gilliam's The Adventures of Baron Munchausen (1989), alongside Robbie Coltrane in Nuns on the Run (1990) and in Casper (1995). He also played Ratty in Terry Jones' version of the The Wind in the Willows (1996). However, his own creative projects - such as the movie Splitting Heirs (1993), a comedy he wrote, starred in and executive-produced - were mostly unsuccessful with critics and audiences.

In recent years, Idle has worked with people who regard him as a huge inspiration, such as Trey Parker and Matt Stone in South Park: Bigger, Longer & Uncut, in which he voiced Dr. Vosknocker. He has also made three appearances on The Simpsons as famous documentarian Declan Desmond, so far the only appearance on the show by a Python. From 1999 to 2000, he played Ian Maxtone-Graham on the NBC sitcom Suddenly Susan. Idle has also acted as Narrator to the AudioNovel version of Charlie and the Chocolate Factory by Roald Dahl. He has recently provided the voice of Merlin in the DreamWorks animated film Shrek the Third, which reunites him with his fellow Python alumnus John Cleese. He was reported to have stormed out of its premiere and has said he may sue the producers of the film after seeing them directly copy a gag from his earlier film Monty Python and the Holy Grail. The gag in question is banging coconuts together to imitate hoofbeats- a running gag throughout Monty Python and the Holy Grail.