They will join the previously announced Amanda Holden as Princess Fiona and Richard Blackwood as Donkey.
Lindsay was last seen on the London stage in Sucker Punch at the Royal Court. Other leading roles include The Homecoming at the Almeida, Under the Blue Sky at the Duke of Yorks, Nathan Detroit in Guys and Dolls at the Piccadilly Theatre and The Pillowman at th National Theatre.
Harman is best known for playing Dennis Rackman in BBC TV's "EastEnders." His previous West End stage roles include Three Days of Rain at the Apollo Theatre, Sky Masterson in Guys and Dolls at the Piccadilly and Privates on Parade at the Donmar Warehouse.
In a press statement, producers Bill Damaschke and Caro Newling commented, "We were already thrilled that our dream of bringing Shrek The Musical to London was coming true, but to do so with a cast led by these charming, funny and incredibly talented actors has exceeded our expectations. Nigel, Amanda, Nigel and Richard all blew us away with their audition performances and we knew that we had found our new London Shrek family! Our search continues for the further 28 cast members who will bring the original Shrek story to life on the Theatre Royal, Drury Lane stage."