Protheroe was born in Salisbury, Wiltshire, to a Welsh father and English mother. He joined a local church choir when he was twelve years old, and started piano lessons at about the same time. The music of Cliff Richard and the Shadows inspired him to start learning the guitar. He joined a rock band called the Coasters (not to be confused with the similarly titled United States outfit) as lead singer in 1961, and also joined an amateur theatre group called the Studio Theatre.

Protheroe joined the folk group Folk Blues Incorporated (FBI) when he was nineteen, while at this time listening to Bob Dylan and the Beatles. He came to London with FBI in 1965, and played in folk music clubs in and around London. In 1966, Protheroe began his career as an actor. His first job was with his local repertory theatre in Salisbury. He worked there for about seven months then spent the next five years in various theatre companies around Britain, developing his musical skills as well as becoming an experienced actor. In 1968 he worked for nearly two years in a theatre company in Lincoln, where he met Martin Duncan who was also a musician, writer and actor. Over the next few years they collaborated on various musical and artistic projects.

In 1973, Protheroe was playing the part of a pop singer in a play called Death on Demand, when a representative from Chrysalis Records heard a song he had written for the show. His first single, “Pinball”, was released in August 1974, and it entered the UK Singles Chart at number 40 and reached a peak of number 22. The saxophone solo was played by Tony Coe. Then followed by an album of the same name. The follow-up single “Fly Now” made Capital Radio’s “Capital Countdown” chart.

A developing acting career found him, in 1976, starring in the London production of the rock musical, Leave Him to Heaven at the New London Theatre. Temporarily laying aside his career in music, he focused on theatre, television and film.

He also has a small role in the 1978 film Superman as a co-pilot of Air Force One.

In 1982, he was cast in the BBC Two play Spider’s Web by Agatha Christie, and in the following year played the role of Edward IV in the BBC Television Shakespeare productions of Henry VI, Part 2, Henry VI, Part 3 and Richard III.

In 1984 he played in the West End musical “Pump Boys and Dinettes” on piano and vocals and percussion.

In 1997, Basta Records released a compilation album of the first three albums from the 1970s, with an additional disc of unreleased material in a box set, Brian’s Big Box. The advent of the internet allowed for a resurgence in interest in his musical career. He released the collection Citysong in 2005. It was described as a “new single with seventeen bonus tracks and two movies”. It included the new song “Holyoke Hotel” as well as two homemade videos. After remastering his original tapes at Abbey Road Studios, EMI studios released a greatest hits collection called Pinball and Other Stories in 2006.

In 2002, Protheroe played the role of Gower in Adrian Noble’s production of Pericles, Prince of Tyre.

In 2007, he was cast as Saruman in the original stage musical version of The Lord of the Rings. Performances began at London’s Theatre Royal, Drury Lane on 21 June 2007. Protheroe played his final performance at the end of his year contract in June 2008.