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, 1945. 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.

The school had begun life as a Victorian orphanage, and during Idle's time was a charitable foundation dedicated to the welfare of children who had lost one or both parents. Its pupils, who were mainly the children of dead English soldiers, still referred to it as the 'Ophney'.

Idle is quoted as saying: "It was a physically abusive, bullying, harsh environment for a kid to grow up in. I got used to dealing with groups of boys and getting on with life in unpleasant circumstances and being smart and funny and subversive at the expense of authority. Perfect training for Python."

Idle attended Pembroke College at the University of Cambridge, where he studied English. At Pembroke, he was invited to join the prestigious Cambridge University Footlights Club by the President of the Footlights Club, Tim Brooke-Taylor, and Footlights Club member Bill Oddie.

I'd never heard of the Footlights when I got there, but we had a tradition of college smoking-concerts, and I sent in some sketches parodying a play that had just been done. Tim Brooke-Taylor and Bill Oddie auditioned me for the Footlights smoker, and that led to me discovering about and getting into the Footlights, which was great.

When Idle joined the Footlights Club, the other members included Graham Chapman and John Cleese, who were also attending the University of Cambridge.

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."

Idle's work in Python is often characterised by an obsession with language and communication: many of his characters have verbal peculiarities, such as the man who speaks in anagrams, the man who says words in the wrong order, and the butcher who alternates between rude and polite every time he speaks.

A number of his sketches involve extended monologues (for example the man who won't stop talking about his unpleasant experiences with holidays), and he would frequently spoof the unnatural language and speech patterns of television presenters. Additionally, like Palin, Idle is said to be the master of insincere characters, from the David Frost-esque Timmy Williams, to small-time crook Stig O'Tracy, who tries to justify the fact that organized crime master Dinsdale Piranha had nailed his head to the floor.

One of the younger members of the team, a year behind Cleese and Chapman at Cambridge, Idle was closest in spirit to the students and teenagers who made up much of Python's fanbase. Python sketches dealing most with contemporary obsessions like pop music, sexual permissiveness and recreational drugs are Idle's work, often characterized by double entendre, sexual references, and other "naughty" subject matter, most famously demonstrated in "Nudge Nudge." Eric Idle originally wrote "Nudge, Nudge" for Ronnie Barker, but it was rejected because there was 'no joke in the words'.

A competent guitarist, Idle composed the group's most famous musical numbers, most notably "Always Look on the Bright Side of Life", the closing number of Life of Brian, which has grown to become a Python signature tune. He was responsible for the "Galaxy Song" from The Meaning of Life and (with Cleese) "Eric the Half-a-Bee", a whimsical tune that first appeared on the Monty Python's Previous Record album.