 Sinclair was born at Cambewarra on the South Coast of New South Wales and was raised on his family's farm.He took an interest in the development of dairy co-operatives in the region and became a successful butter factory manager, at the Bengelala Dairy Company and Foley Bros. Cambewarra factories, then Foley Bros. creamery in Sydney, before becoming district manager of the Australian Dairying Company Ltd in the Manning River area for three years.In 1901, he relocated to Ipswich in Queensland as manager of the Queensland Farmers Co-operative Dairy Company, later expanding its operations across the region.