Born in Scotland, Sinclair went to Oregon in 1832 with the Nathaniel Wyeth expedition, and a year later boarded a whaling boat for Hawaii where he became a partner in a Honolulu mercantile firm. He arrived in Monterey in December 1839, and during the following year settled at John Sutter’s then-fledgling trading post on the American River. In 1842 he took possession as manager of the Rancho del Paso across the river from Sutter’s Fort as a representative of Eliab Grimes, who owned the property. Sinclair was the alcalde (mayor and justice of the peace) of the Sacramento District from September 1846 to September 1848, officiating at several pioneer marriages and taking an administrative role in the rescue of the snow-stranded Donner Party. He died in 1849 on board a ship bound for the eastern United States, just as thousands were swarming west for the gold fields.