Masson is an Artistic Associate with the RSC and was in the Royal Shakespeare Company acting ensemble from 2003 to 2011. His roles included Horatio to Toby Stephens' "Hamlet" in 2004, and Feste in "Twelfth Night" in 2005. Both productions were directed by then RSC Artistic Director Michael Boyd. He also played Porter in Dominic Cooke's "Macbeth" and Dromio of Ephesus in Nancy Meckler's "Comedy of Errors". He played Judas in "Pilate Project". He performed in Boyd's critically acclaimed "Histories" cycle in Stratford-Upon-Avon and at The Roundhouse, Camden, in 2006–2008. His roles in the cycle included Bagot in "Richard II", Rumour in "Henry IV, Part Two", Chorus in "Henry V" and King Edward IV in "Henry VI" and "Richard III". In 2009 he played Samson in Boyd's production of "The Grain Store". He played Romeo opposite Kathryn Hunter's Juliet in Ben Power's adaptation of "Romeo and Juliet" called "Tender Thing". He played Merlin in Greg Doran's production of Mike Poulton's "Morte d'Arthur". He played Gena in "Ahasverus" as directed by Michael Fentiman, at the Hampstead Theatre. He later appeared as Jacques in Boyd's "As You Like It", and Friar Lawrence in Rupert Goold's production of "Romeo and Juliet". Both plays came to the Roundhouse in Camden in late 2010. They played Park Avenue Armory in New York in summer 2011.