At the age of eighteen Ian played in the orchestra for the musical "Hair" and studied percussion at The Guildhall School Of Music under teacher Gilbert Webster.

His first professional band was Darryl Way's Wolf.

He played for Gordon Giltrap and on the 1975 album Birds by Dutch band Trace.

Ian toured extensively with ex-Genesis guitarist Steve Hackett from 1980 to 1984, as well as appearing on several of Steve's albums, including Highly Strung, Till We Have Faces, Time Lapse Live, and Feedback.

Ian joined Marillion in 1984 after a long search for a replacement for drummer Mick Pointer, who had left the band in 1983. He was the fifth drummer to play for Marillion.

Music critic John Franck stated Mosley's "spot-on drumming was the perfect foil for Marillion's meticulous musicianship".

In 1996, Ian teamed up with French guitarist Sylvain Gouvernaire to form the band Iris (also featuring Pete Trewavas on bass) and to record the instrumental album Crossing the Desert.

In 2001, Ian joined saxophonist Ben Castle, son of entertainer Roy Castle, and they recorded a jazz-themed album together, Postmankind. The album also featured guest performances by John Etheridge, Steve Hackett and Marillion members Steve Rothery and Pete Trewavas.

Mosley played on fellow Marillion bandmate Pete Trewavas' and Eric Blackwood's Edison's Children album In The Last Waking Moments..., playing drums on the epic 16 minute "The Awakening".

In 2019, Ian (in collaboration with Georgina Wistow) published a critically acclaimed collection of his memoirs entitled "Do I Owe You Money?".