Lockhart began as a stage actor, working with the Royal Shakespeare Company, the National Theatre and others. Since 1998, he has acted mostly with Propeller, an all-male theatre company of which he is now an associate director. He is also an associate of the Teatre Akadèmia Theatre Company in Barcelona and has directed Romeo and Juliet and As You Like It in Catalan, using new translations by Miqel Desclot. He teaches and directs at drama schools in London, including the Royal Central School of Speech and Drama, LAMDA, and the Italia Conti Academy of Theatre Arts.He played David Cameron in The Three Lions, a comedy written by William Gaminara, a role for which he was nominated as best actor by The Stage at the Edinburgh Festival of 2013. Lockhart returned to the role when the play was later staged at the St James Theatre, London, in 2015, and stayed with the production when it moved on to the Liverpool Playhouse.He is now an author of a handbook for actors called Heavy Pencil, and his newest book, The Lizard.