Valentine began his career in 1981 working alongside composers such as Stephen Sondheim, Stephen Schwartz, John Kander, Andrew Lloyd Weber, and Maury Yeston.

He has conducted the following West End musicals (all of which won Olivier Awards for Best Musical or Best Musical Revival): 42nd Street (Drury Lane), Kiss Me Kate (Victoria Palace), Merrily We Roll Along (Donmar Warehouse), Chicago (Adelphi Theatre), and City of Angels (Donmar Warehouse). He has most recently conducted My Fair Lady at The London Coliseum.

He has conducted for many international artists including: Barbara Cook, Eartha Kitt, Betty Buckley, Elaine Paige, Liza Minelli, Christopher Reeves, Dame Judi Dench, Sir Bryn Terfel, Catherine Zeta-Jones, Chita Rivera, Jerry Lewis, Sir Roger Moore, Sir Ian McKellen, Clarke Peters, and Victoria Wood.
Gareth’s Requiem has been performed at worldwide, and was recorded at Abbey Road Studios, London.
He was commissioned by The Old Vic to compose a new score for Aladdin starring Sir Ian McKellen as Twankey.

He composed the ballet Strictly Gershwin for English National Ballet which played at the Royal Albert Hall and a sellout UK tour with seasons in Tulsa and Queensland.