Jacob was born on Earth in the city of Belfast, County Antrim, Ireland. He was the only child of an Irish woman from Carrickfergus, and an American of Scots-Irish descent named Sinclair. Though primarily raised in Ireland, Jacob spent the summers of his youth on his father's family ranch in Colorado. His father died when he was in his early teens, and his mother shortly after he joined Starfleet. The last surviving member of both families, Jacob inherited the ranch, a home and property in the north of Ireland and ancestral land of the Clan Sinclair in Scotland. He also kept a flat in San Francisco.
Late in his career, Sinclair was an operative for Starfleet Intelligence, and worked with Mark Coleman, Elias Vaughn and Ian Troi. According to official reports, Sinclair was killed in a shuttle accident near Christopher's Landing, Titan on 22 June, 2341, just before the birth of his son, Timothy. The mystery surrounding his death would haunt his son for many years. (Star Trek: Pendragon)