Born in Goshen, Guysborough, Nova Scotia, Sinclair was educated in the Common School of Goshen, Guysborough Academy and Dalhousie College, Halifax.
He was elected to the Canadian House of Commons for Guysborough in a 1904 by-election after the sitting Member of Parliament, Duncan Cameron Fraser, was appointed a Judge of the Nova Scotia Supreme Court. A Liberal, he was re-elected in 1904, 1908, 1911, and 1917.
Liberal Party of Canada, Nova Scotia Liberal Party.
A lawyer, he was Mayor of New Glasgow in 1890-1891 and member of the Nova Scotia House of Assembly from 1894 to 1904.