Served on board the USS Pontoosuc during the capture of Fort Fisher and Wilmington, 24 December 1864, to 22 February 1865. Carrying out his duties faithfully throughout this period, McWilliams was so severely wounded in the assault upon Fort Fisher that he was sent to the hospital at Portsmouth, Va. McWilliams was recommended for his gallantry, skill and coolness in action while under the fire of the enemy.