From 1970 to 1974, Hayes was director of the London Museum, overseeing its move to new premises on London Wall. He was subsequently appointed director of the National Portrait Gallery, succeeding Sir Roy Strong in the post. The duties that came with the NPG directorship did not curtail Hayes's scholarly activities, however, and during his tenure he managed to curate a major Gainsborough exhibition at the Tate and the Grand Palais, Paris), and numerous smaller ones, including one of Graham Sutherland's work. Under his aegis the NPG's annual portrait award (now the BP Portrait Award) was established in 1980, and two regional outposts for the gallery opened at Beningbrough Hall in Yorkshire and Bodelwyddan Castle in Denbighshire. Plans to move the institution to larger premises in London were never realised, although a new block for staff was built across the road from the Gallery's Victorian building. After his retirement in 1994 Hayes continued in his scholarly work, curating a further Gainsborough exhibition in Ferrara and serving as vice-president of the Walpole Society.