John Mitchell Sinclair (c. 1819 – 7 October 1890) was a businessman and politician in the colony of South Australia. Sinclair arrived at Port Adelaide about 1849, and after serving in the Customs acted as clerk to Captain Scott. In 1861 he went into partnership with Joshua Little (c. 1831–1920) as Sinclair & Little, customs and shipping agents. Little and Sinclair dissolved their partnership in January 1865, and Sinclair went into business for himself, using the same warehouse; Little went into partnership with his brother-in-law David Wylie Scott (1833–1887).