During the 1990s, he worked with such comedians as Simon Pegg, Mark Heap, Julia Davis, Amelia Bullmore and others in the sketch-comedy series Big Train.

[4] He had previously worked with some of the cast in the satirical series Brass Eye, and the dark comedy Jam, both written by Christopher Morris.

In 2004, Eldon appeared in the BAFTA award-winning dark comedy Nighty Night as Terry Tyrrell, husband to Julia Davis' character.

Eldon has appeared on stage and screen numerous times with the comedian Bill Bailey; they performed as a spoof of the German band Kraftwerk, singing German versions of the "Hokey Cokey" and The Wurzels song "The Combine Harvester" for the recorded version of Bill Bailey's Part Troll comedy tour, along with two others (John Moloney and Martin Trenaman).

In late 2006, Eldon, alongside Bill Bailey, helped to organise and produce a short tour and a West End run of Pinter's People at the Haymarket Theatre, London.

Eldon has also had minor guest starring roles in numerous comedy projects, including Smack the Pony, Green Wing, The IT Crowd and The Kennedys.

[8] Regular characters included the obnoxious poet Paul Hamilton; Stanley Duthorpe, the fictional man from the North of England; and an offensive French musician.

[4] In October 2008, he played the part of pessimistic, unpopular Big Brother housemate Joplin in Charlie Brooker's five-part E4 horror thriller, Dead Set.

He has also appeared in several minor serious roles in British drama shows such as Robin Hood, Utopia, Merlin, Skins, New Tricks and Hustle.

In 2016, he appears in the 6th season of the American fantasy drama series, Game of Thrones along with Richard E. Grant as part of a theatre troupe, with Eldon's character portraying a version of Ned Stark.

Eldon played the role of Pete in the David Shrigley/Chris Shepherd animation, Who I Am And What I Want, and also of Mick McManus in Tim Plester's short film World of Wrestling in 2007.

In 2009, he played the lead role of Arthur in Radio Mania: An Abandoned Work, a stereoscopic 3D film for the BFI, directed by British artists Iain Forsyth and Jane Pollard.,[10] Eldon co-authored the theme tune, and is script editor for, the children's series Genie in the House.

In 2008, Eldon presented Poets' Tree, a four-part "poetry type programme" for BBC Radio 4, in the character of Paul Hamilton, which was co-written and edited by Stewart Lee.

Eldon has also written and starred in a six-part series of monologues as people from different professions, collectively entitled Speakers, broadcast on the London art radio station Resonance FM.

[18] Between 2009 and 2013, Eldon appeared as both Dean the Dwarf and Kreech, the "Right Hand of Darkness" in the BBC Radio 4 parody of the Lord of the Rings Trilogy called ElvenQuest.