It was during his time in Ohio that war with Mexico broke out and George volunteered his services.

McGinnis and the 11th Indiana took part in Wallace's raid on Romney, West Virginia, on 13 June 1861.

During the Battle of Fort Donelson, his regiment was temporarily attached to Lew Wallace's division and fought in the counterattack on the Union right.

On 4 April 1863, McGinnis was promoted to brigadier general of volunteers, to rank from 29 November 1862.

McGinnis's brigade was heavily engaged in the Battle of Champion Hill and fought with distinction.

He moved to Indianapolis and ran a fiduciary business and served in various local political positions.