[1][3][4] As an officer with Major General Zachary Taylor's forces, Maclay was wounded at the Battle of Resaca de la Palma, Texas, on May 9, 1846, at the start of the Mexican–American War.

[1] Maclay resigned as captain in the U.S. Army on December 31, 1860, in order to manage his Louisiana properties.

[1][2] Holmes assigned Maclay to the staff of Major General John George Walker, commander of an infantry division from Texas, on January 2, 1863.

[1] Maclay was assistant adjutant general and inspector general for Walker and later became chief of staff for the division, which fought in Louisiana and Arkansas at the Battle of Milliken's Bend, Red River Campaign, and Battle of Jenkins' Ferry in 1863 and 1864.

[2][7] Smith's appointment of a staff major to brigade command over various colonels caused dissatisfaction despite Maclay being held in high regard by his men.

[4][6] In January 1865, Smith gave Maclay a 60-day leave of absence in order to deal with the situation.

[6] He lived near New Roads, Louisiana, in Pointe Coupee Parish and was active in the Episcopal Church.

[6] Robert Plunket Maclay died on May 20, 1903, at the home of his daughter, Levy Plantation in Pointe Coupee Parish.