Mostyn trained at Mountview Theatre School.
Most recently Mostyn has been touring the UK as Resident Director on Fiddler on the Roof. Other theatre credits include: understudied and played Emile de Becque in South Pacific (national tour); Bobby in Cabaret (English Theatre, Frankfurt); Goddard in Let Him Have Justice (Cochrane Theatre, London); Prince Charming in Cinderella (Grand Theatre, Blackpool); understudied and played Adam Pontipee in Seven Brides for Seven Brothers (Theatre Royal Haymarket, London); Dromio of Ephesus in The Comedy of Errors (national tour); Viceroy in Naked Flesh (Arts Theatre, London); Johnny in Circus Agogo (New End Theatre, London); Caleb in Seven Brides for Seven Brothers (national tour); Cecil C in Jack and the Beanstalk (Grand Theatre, Wolverhampton); Gaveston in Edward II (Oval Theatre, London); Charles in Me and My Girl (Devonshire Park Theatre, Eastbourne); Sammy in Blood Brothers (Phoenix Theatre, London); Avrahm in Fiddler on the Roof (national tour); Wolf in Babes in the Wood (Theatre Royal, Bath); and Annas in Jesus Christ Superstar (English Theatre, Frankfurt). Mostyn has worked on 12 shows as resident director, throughout Europe, for Openwide International.
Recording and television credits include: Let Him Have Justice (TER); Blackpool (BBC); A Decline of Magic (Wodum); South Pacific (BBC Radio 2); and commercials for Pilsner and Honda. Cabaret and concert credits include: South Pacific (Symphony Hall, Birmingham); Jerry Herman (Broadway Theatre, London); Bobby and Victor (English Theatre, Frankfurt); Too 2 Much (Soho Revue Bar, London); So Far (Dolman Theatre, Newport); Celebration (Queen’s Theatre, London); Bits and Bobs (Theatre Museum, London); Showtime (Dolman Theatre, Newport); The Talk of London (New Theatre, London).