Grandage, who was artistic director of Donmar Warehouse from 2002 to 2012, has in the last year overseen West End revivals of John Logan's Tony Award-winning play Red and Martin McDonagh's The Lieutenant of Inishmore as well as the Broadway production of Disney's Frozen. Previous shows have included Peter and Alice with Ben Whishaw and Judi Dench and Photograph 51 with Nicole Kidman.
The award was presented to him by The Lieutenant of Inishmore's star Aidan Turner at the Prince of Wales Theatre on Sunday 3 March, with the award recognising Grandage's contributions both on stage and to fostering new theatre talent.
In 2013, Grandage formed his charity MGCfutures, dedicated to supporting the work of young theatre makers and theatre audiences of the future. The charity's patrons include Judi Dench, Nicole Kidman, Jude Law and Daniel Radcliffe.
Law and Dench were among those celebrating the work of Grandage in a specially recorded video, which also featured interviews with award-winning actors Eddie Redmayne and Felicity Jones, the Bush Theatre's artistic director Lynette Linton and playwright Isley Lynn.
You can read the full list of WhatsOnStage Award winners here.