Sinclair was born in New York (North America), son of James Sinclair and Elise Morrell, belonging to a family of pioneers. He arrived at the port of Buenos Aires at an early age, enlisting in the Navy of William Brown, taking part in the Brazilian War. Finished the war of Brazil was promoted to Captain in 1830, serving as Comandante of the sumaca . In 1831 he served temporarily as Captain of the pontoon schooner In 1832 Sinclair was promoted to Sergeant Mayor.