After college, he joined an improv group with Mike McShane. Both Proops and McShane impressed producers Dan Patterson and Mark Leveson, who put them on their show, "Whose Line Is It Anyway?". He frequently instigated jokes concerning various idiosyncrasies and differences between British English and American English, and would frequently banter with Clive Anderson on these matters (among others). He lived in London for four years when he was doing the show and lists McShane, Richard Vranch and Colin Mochrie among his best friends. After the show ended, he was recalled for the American version and has been a frequent "fourth contestant". Through 2020, he has appeared in 74 U.S. "Whose Line" episodes, more than any other non-regular cast member.