Sinclair won widespread acclaim for his first book of history, "The Origins of the Maori Wars" (1957). His next book, "A History of New Zealand" (1959), is often regarded as a classic in New Zealand history. The book remains in print, being revised several times, the last, with additions by fellow academic Raewyn Dalziel, in 2000. In 1967 he founded the "New Zealand Journal of History".