[3][4] In 1966, Sinclair and fellow lecturer Bob Chapman established The University of Auckland Art Collection, beginning with the purchase of several paintings and drawings by Colin McCahon.
In both his poetry and his work as a historian, Sinclair was a nationalist, in the sense that he was concerned with forging a national identity for New Zealand that was independent of its colonial origins.
[5]<[6] Later he wrote an acclaimed biography of Labour Prime Minister Walter Nash who had left his vast personal archives at Sinclair's disposal.
[3] In the 1983 Queen's Birthday Honours, Sinclair was appointed a Commander of the Order of the British Empire, for services to literature.
[7] Two years later, he was made a Knight Bachelor, for services to historical research and literature, in the 1985 Queen's Birthday Honours.