Born in Goshen, Guysborough, Nova Scotia, Sinclair was educated in the Common School of Goshen, Guysborough Academy and Dalhousie College, Halifax.

A lawyer, he was Mayor of New Glasgow in 1890-91 and member of the Nova Scotia House of Assembly from 1894 to 1904.

He was elected to the House of Commons of Canada for Guysborough in a 1904 by-election after the sitting MP, Duncan Cameron Fraser, was appointed a Judge of the Nova Scotia Supreme Court.

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