Born in Cleveland, Ohio, he began writing cartoons for the Cleveland Press. After that, he moved on to Hollywood, California and continued his writing career with work on Get Smart. Then, in 1968, he won an Emmy for his work on Rowan & Martin's Laugh-In. Following this, he moved on to Marcus Welby, M.D., The Sonny and Cher Comedy Hour, Police Woman, The Waltons, and CHiPs. In the 1980s and '90s, he worked on a scattering of television shows and movies, including co-writing the Super Mario World episode "The Wheel Thing" and serving as a main writer for the second season (1985-86) of the cartoon series Inspector Gadget.
Hanrahan left Beverly Hills for Eureka in northern California in 1992. Following a garage fire, however, he was evicted, and all of his possessions were seized by the landlord, including his Emmy. After losing his wife in 2004, he began to wander the streets. Jack Riley, an old friend, tried to help Hanrahan, but was unsuccessful. After friends in Eureka bought him a bus ticket, he came back to his hometown of Cleveland, where he had been homeless until his death.
Hanrahan was featured on the front page of the (Cleveland, Ohio) Plain Dealer on March 22, 2007, detailing his homelessness.