He entered West Point with his kin Langdon C. Easton of St. Louis, who was Chief Quartermaster of General William T. Sherman's 100,000-man army.

Smith was engaged on active service on the frontier in the Southwest and in the Mexican–American War, in the latter briefly commanding the Mormon Battalion.

In retaliation for Forrest's raid on Fort Pillow, north of Memphis, Union General Andrew Jackson Smith, with a large military force, arrived in Oxford, Mississippi on August 24, 1864, and in one day burned the Lafayette County Courthouse, all the business houses on the Square, except one, and all homes in the immediate area.

Smith was then summoned to join forces with Maj. Gen. George Henry Thomas at Nashville, Tennessee, then threatened by the advance of Confederate Lt. Gen. John Bell Hood.

Smith bore a conspicuous share in the crowning victory at the Battle of Nashville leading his troops past the Confederates' south flank.

[6] The Senate reconfirmed this appointment on July 14, 1866, after recalling the confirmation and return of the nomination to President Johnson for possible readjustment of the date.