Grimsby may have fought at the Battle of Northampton in July 1460, and certainly fought at the Battle of Wakefield in December, a Lancastrian victory, and at the Battle of Towton the following March, a catastrophic defeat. He was attainted by the Yorkists, his lands were confiscated and awarded to John Ferriby, and Grimsby fled into obscurity. He was reported to have died in the Marshalsea of "black jaundice" in 1462, and his wife was allowed to take over her own inherited estates in August 1464.