Turnbull raised and commanded the New York Volunteers in January 1776; this regiment was renumbered in May 1779 as the 3rd American Regiment. He was based at Rocky Mount, North Carolina and commanded at the Battle of Rocky Mount, Lancaster County, South Carolina on August 1, 1780. Captain Christian Huck of Huck’s Defeat (July 12, 1780) served under Turnbull's command.