Egan's career began on stage, in 'Charlie Girl'. His first television role came in the 1968 production of Cold Comfort Farm. He first hit the headlines the following year when he played the gangster Hogarth in 'Big Breadwinner Hog'. In the 1970s, he featured in 'The Love School and 'Lillie' before going on to play the role of King George IV in 'Prince Regent' in 1979. He also played minor roles in smash hits 'Chariots of Fire' in 1981, 'To Catch A King' in 1984 and 'Bean' in 1997.
In addition to his roles in Ever Decreasing Circles, and appearances as Hugh "Shrimpie" MacClare, in Downton Abbey, Egan has also narrated the video games Forza Motorsport 3 and 4.
Outside of the acting world, Egan is an avid advocate of animal rights. He acts as an ambassador for Sarajevo dog rescue group Saving Suffering Strays, as well as the Animals Asia Foundation. Egan is a vegan and refused to be shown with meat on his plate during the filming for Downton Abbey.