He was the first Director of the Whitechapel Art Gallery from 1901 to 1911, and became Keeper at the Tate Gallery in 1911. In 1917 he changed his title from "Keeper" to "Director". Sir John Rothenstein described Aitken as "an ordinary man: his intelligence was relatively pedestrian, his powers of self-expression scarcely adequate". However, Rothenstein considered that the job "brought out qualities that made him a great director: clarity and firmness of purpose".