The 33rd Illinois participated in the Mobile campaign on 18 March–12 April 1865. The regiment was assigned to Colonel William L. McMillen's 1st Brigade, Brigadier General John McArthur's 1st Division, Major General Andrew Jackson Smith's XVI Corps, Major General Edward Canby's Army of West Mississippi. McArthur's Division engaged in the Battle of Spanish Fort starting on 27 March. On the night of 8 April, the Confederate defenders evacuated Spanish Fort. Smith planned to have McArthur's 1st and Carr's 3rd Division at Spanish Fort join Brigadier General Kenner Garrard's 2nd Division for the Battle of Fort Blakeley on 9 April, but the fort was quickly captured by assault and they were not needed.