Mulrooney competed on Star Search and lost, but it helped his early career, since he was hired to work for the program, writing comedy and doing audience warm up.
Upon relocating to Los Angeles, California, Mulrooney became a "regular" at The Improvisation, The World Famous Comedy Store and The Laugh Factory.
After Joan Rivers left as host of The Late Show on the Fox network in 1987, Mulrooney was asked to replace her.
Mulrooney hosted Comic Strip: Live, first for a year as a local show on KTTV in Los Angeles in 1988, continuing when Fox TV programming executives expanded the show to Fox-owned stations, then network-wide on Saturday nights, for an additional year.
In 1994 Mulrooney wrote, and starred, for two seasons in a sitcom called Midtown North which aired on Comedy Central.