Jacob studied law with Theodore Sedgwick, settled in Windsor, Vermont in 1780, and attained admission to the bar. He joined the militia during the American Revolution, and served in the company commanded by Captain John Mercy during a three-day mobilization in March 1781. The militia in eastern Vermont had been called out by General Jacob Bayley of Newbury in response to a report of British troops from Canada operating in the Newbury area, but the report proved unfounded.