[3] He has been a producer at the annual Edinburgh Fringe Festival and is responsible for several of its productions including Twelve Angry Men in 2003, The Odd Couple in 2005 and Morecambe in 2009, and The Shark Is Broken in 2019.

The Shark is Broken also transferred to London's West End and won a Laurence Olivier Award nomination for Best New Comedy which opened on Broadway on August 10, 2023 for an 18 week run.

His first professional job was understudying the roles of both Mick and Aston in The Caretaker at the LA Theatre Center, directed by Alan Mandell for which he received his Equity Card.

After performing in a number of plays in Los Angeles, including The Elephant Man by Bernard Pomerance, The Yorkshire Tragedy – a disputed Shakespeare text, City Gents by Ron Hart The Private Ear & The Public Eye by Peter Shaffer, he returned to London to study at LAMDA – The London Academy of Music and Dramatic Art – on the Postgraduate Course, graduating in 1990.

It played at the Theatre Royal, Haymarket in London's West End for nine months, during which Masterson committed Under Milk Wood by Dylan Thomas to memory for his 2nd solo performance.

Both Under Milk Wood and Playing Burton achieved critical success and transferred to Riverside Studios London, and from there, both shows toured around the UK for several years.

In 1996, Masterson directed a production of The House of Correction by Norman Lock for the Edinburgh Fringe and presented it at the Pleasance where it received a Stage Award for Best Actress[5] for Beth Fitzgerald.

Stage Award nominations for Best Actress were also received by Madeleine Sami (for Number Two) and Jules Leyser for All Words For Sex which Masterson directed.

Masterson also received a Stage Award Nomination for Best Actor for his solo Under Milk Wood now with music added by composer Matt Clifford a multi-year collaborator with The Rolling Stones.

The show went on to break box office records at the Edinburgh Fringe Festival, two West End runs, and a Number One U.K. tour with Shane Richie taking over from Slater.

[10] In 2005, Masterson directed the most successful ever theatre production at the Edinburgh Fringe Festival, The Odd Couple by Neil Simon starring comedians Bill Bailey, Alan Davies, Phil Nichol, Owen O'Neill, Dave Johns, Ian Coppinger, Katherine Jakeways and Lizzie Roper.

In 2008, he directed Justin Butcher in his performance of his own Scaramouche Jones[8] which sold out its run at the Assembly Rooms and embarked on a national tour.

In 2012 Masterson was invited to direct and play the role of Oscar in Neil Simon's The Odd Couple at the Theatre At The Mill with actor and comedian Dave Johns.

In 2017, Masterson wrote, produced & directed Michael Brandon (Dempsey & Makepeace) in Off Ramps at Edinburgh Fringe Festival to great acclaim.

For the festive season he premièred his 11th solo work, Charles Dickens' A Christmas Carol [16] adapted and directed by Nick Hennegan.

In 2019, Masterson collaborated with actor/writer Ian Shaw and writer Joseph Nixon, to present and direct The Shark Is Broken - a play about the making of Jaws.

The show was the hit of the 2019 Edinburgh Fringe and was subsequently picked up by Sonia Friedman Productions for a West End transfer in May 2020, but postponed due to the Covid Pandemic.

Early 2023, Masterson directed Peter Tate in Picasso: Le Monstre Sacré [19] by Terry D'Alfonso at The Playground Theatre, London.