Edmondson is probably best known as the anarchic punk Vyvyan in the cult BBC sitcom The Young Ones (1982-1984) and later, for his role as Eddie Hitler in the BBC sitcom Bottom (1991 - 1995) which he also co-wrote. In both of these roles he starred alongside comedy partner Rik Mayall.

He is the second of four children. Though born in Bradford, as a child he lived with his family in a variety of places including Cyprus, Bahrain and Uganda.

He attended public school in Pocklington, Yorkshire, and later went onto the University of Manchester to study drama. Edmondson met Mayall while studying at the University. They soon became friends and before long found work on the burgeoning alternative comedy scene. Under the name "Sir (or occasionally Lady) Adrian Dangerous" Edmondson gained popularity as one half of The Dangerous Brothers alongside Mayall. The pair became one of the star attractions at the The Comedy Store. As their populatity grew, they and some other popular comedians (including Alexi Sayle, Peter Richardson, Nigel Planer and French and Saunders) split away from the Comedy Store to set up their own venue - The Comic Strip club.

The Comic Strip Club soon gained a reputation as one of the most popular comedy clubs in London and soon came to the attention of Channel 4. Edmondson and the others were commissioned to act in six self-contained half-hour films, using the group as comedy actors rather than standup performers. The series, entitled The Comic Strip Presents... debuted on 2 November 1982 (the opening night of Channel 4). The first episode to be broadcast was "Five Go Mad In Dorset", a parody of The Famous Five, which drew anger from some viewers for the way it mercilessly satirised a children's classic. Edmondson starred as one of the five.

At the same time as The Comic Strip Presents... was being negotiated, the BBC signed up Edmondson, Mayall, Richardson, Planer and Sayle to star in The Young Ones, a sitcom in the same anarchic style as The Comic Strip. The show revolved around the shared house where the students lived during their study at Scumbag College. It was noted at the time of its first airing for its violent slapstick, with Edmondson's character as the main instigator, and this is a trait which has stayed with him throughout his career. The series captured public imagination and remains one of Britian's most popular sitcoms.

Following the success of The Comic Strip Presents... and, to a greater extent, The Young Ones, Edmondson and Mayall returned to their "Dangerous Brothers" characters in Saturday Live (1985-1987).

In 1985 Edmondson married fellow Comic Strip performer Jennifer Saunders. The couple now have three children Eleanor, Beatrice, and Freya. Edmondson's university nickname of "Eddie Monsoon", a play on his surname, inspired the name of Saunders' character Edina Monsoon on Absolutely Fabulous and his own characters Eddie Catflap (Filthy, Rich & Catflap) and Eddie Hitler (Bottom). Their joint production company is called Mr. and Mrs. Monsoon Limited.

In 1986, Edmondson achieved a number one hit in the UK singles charts when he and his co-stars from The Young Ones teamed up with Cliff Richard to record a new version of Living Doll for the inaugural Comic Relief campaign. Despite having been killed off in the final episode of the series, Edmondson played Vyvyan one last time in the video. The same year he co-wrote the book How to be a Complete Bastard together with Mark Leigh and Mike Lepine.

In 1987 he reunited with Planer and Mayall to star in Filthy, Rich and Catflap. Like The Young Ones, the series was written by Ben Elton and saw Edmondson display the same slapstick characteristcs as Vyvyan, but was closer in personality to his later characer "Eddie Hitler" in Bottom. The series received critical acclaim but poor viewing figures and was cancelled after one series.

In 1989 Edmondson made a memorable apperance in an episode of Blackadder Goes Forth as The Red Baron, arch-nemesis to Mayall's character Lord Flashheart.

In 1991 he teamed up with Mayall once more, this time co-writing and co-starring in their own sitcom Bottom. Edmondson starred as "Edward Elizabeth Hitler" opposite Mayall's "Richie Richard". The series featured the slapstick and crude humour the two had become famous for, but also more in-depth character analysis. Mayall and Edmonson have said Bottom was aimed to be more than just a series of toilet gags but a cruder cousin to plays like Waiting for Godot, about the pointlessness of life. The series proved a hit and is often considered to be the most violent and profane example of the Britcom genre. However, it was critised for it's often vulgar humour. Edmondson was also critisied for essentially repraising the same character he had been playing all of his career. Bottom was also incarnated into five UK stage tours (1991, 1995, 1997, 2001, 2003). The violent natures of these tours saw both Edmondson and Mayall ending up in hospital.

In 1993 Edmondson starred alonside Richard Briers in a controvercial black comedy called If You See God, Tell Him. Edmondson played Gordon Spry, whose Father (Briers) is paralysed and has a greatly reduced attention span. His erratic behaviour causes problems for Gordon. The series was comprised of four episodes, each 45 minutes long, and only broadcast once. The BBC has refused to re-broadcast the series

In 1996, he played the role of Ace Face/Bellboy at The Who's performance of Quadrophenia at London's Hyde Park.

Following the 1997 Bottom stage tour - Hooligans Island - Mayall and Edmondson worked on the idea of a Bottom film in which Richie and Eddie run a hotel. The two had been working on the script together, and were set to co-direct. However, in 1998 Mayall was seriously injured and spent a few days in a coma. Edmondson found this time very difficult. When Mayall came around he helped Edmondson complete the script whilst still in hospital, but it was decided that he was still not fit enough to direct, so Edmondson took the sole directing duites on what became Guest House Paradiso. Since the 2003 Bottom tour Edmondson has shown a lack of interest in working with his friend Mayall, claiming he is more difficult to work with since his accident.

In 2004 he starred in a differant comedic role, as an incompetent surgeon in Doctors and Nurses and had a recurring role in Jonathan Creek.

In 2005 he was one of the celebrities taking part in Comic Relief does Fame Academy (in which he was known for his punk-style version of the song "Can't Take My Eyes Off You") and came in third places.

He has also made three appearances to date in Holby City, starring in a semi-serious role as a doctor who usually works in Ghana.