Shearman was educated at Reigate Grammar School (where he was a contemporary of David Walliams) and University of Exeter. During this time, he was regularly seen on stage at the University in various productions, and made his broadcasting debut as the reporter Fergus McIntyre, reporting on University Radio Exeter (almost live) from Zagreb at the 1990 Eurovision Song Contest.

An established theatrical playwright, Shearman has worked with Alan Ayckbourn, had a play produced by Francis Ford Coppola, and has received several international awards for his work in theatre. His plays include Fool to Yourself, which premiered at the Stephen Joseph Theatre in 1997.

His association with Doctor Who began with a play written for BBV Audios, Punchline, in which Sylvester McCoy played the Dominie, a disguised version of the Seventh Doctor. This was penned under the pseudonym "Jeremy Leadbetter" (the name of a character from the popular BBC sitcom The Good Life). Several audio plays for Big Finish followed. He has also had Doctor Who short stories published.