[1][2] In 1915, the Port Melbourne Standard wrote that he was "by trade a flint glass maker" but did "not now follow that calling, as he is a man of independent means".

He served on Port Melbourne City Council from 1906 until his death, but reportedly declined the mayoralty as it came with an expectation of appointment as justice of the peace, and Sinclair did not wish to pass sentence on people.

[5][6][7][8] In 1915, Sinclair was elected unopposed to the Victorian Legislative Assembly as the endorsed Labor candidate in a by-election for the seat of Port Melbourne following the death of MP George Sangster.

[10][11] He later unsuccessfully contested the 1920 election as a "Democratic" candidate on a platform of free medical services for the poor, having the Chief Justice of the Supreme Court fill the role of Governor to save money, state-controlled railway works and granaries, free primary, secondary and tertiary education and the abolition of the Legislative Council.

[6][7][13] Flags were flown at half-mast at Victorian Football Association games that weekend, and a memorial tablet was later installed at the Port Melbourne Town Hall.