[citation needed] The son of Patrick and Nancy Walsh and the second oldest of four children, Sebastian grew up in Rochdale, Lancashire and showed a precocious talent as a dancer, performing in community theatre as a young child.

He was then Dance Captain on the Broadway production of The Who's Tommy (1993) for Des McAnuff and Wayne Cilento where he played Pinball Lad 1 and various other roles.

[2][failed verification] Commercial work as a choreographer on TV specials, Award Shows and pop videos for the following artists: Destiny’s Child, Shaggy, Ricky Martin, Robbie Williams, Emma Bunton and Bjork.

He directed the film for PBS in America with additional footage shot at the Savoy Hotel where Sinatra often stayed in London.

In 2007 Sebastian opened Kilworth House Theatre and remained as Artistic Director until 2015 directing twelve Landmark musicals.

He collaborated with Stephen Schwartz on a radical re-staging of Pippin placing the action in the World of Video games it opened at the Menier Chocolate Factory in 2011.

La Novicia Rebelde[7](The Sound of Music) was premiered in Bogota, Colombia in 2015 in a new production directed by Mitch Sebastian.

UK Tour and Vaudeville Theatre, West End Por Siempre Navidad - Co-book writer and director of this new Christmas musical for MISI Producciones in Bogota, Colombia The Opera Show[10][11] - Conceived and created by Sebastian early productions at Kilworth House Theatre spawned a World Tour; opening in the USA closing at the Teatro Arcimboldi, Milan, Italy.

Created by Sebastian this large scale multi-media, promenade production opened in a purpose built space in Shenzhen, China.