Postle is an accomplished British author. He served as a curator at the Tate Collection, and Head of British Art to 1900. He has published books on British art from the 18th to the 20th century. In 2004, Postle served as a curator for Tate Britain's exhibit Year of the Garden which marks the bicentenary of the Royal Horticultural Society in London. Prior to joining the Tate, Postle served as Director of the University of Delaware's London Centre and an Associate Professor of Art History for 5 years.