He was promoted to captain in 1847, and saw action at Veracruz, Cerro Gordo, Contreras, Churubusco, and the capture of Mexico City, Matamoros and the Galajara pass.

After Porter's court-martial, Roberts was briefly reassigned to Acting Inspector General of Pope's Department of the Northwest before being recalled to Washington in February 1863.

In 1864, he commanded the District of Carrollton, Louisiana, as part of the XIX Corps and ended the year as Chief of Cavalry for the Union Army's Department of the Gulf.

He was subsequently sent to oversee the District of West Tennessee in 1865, where on March 13, 1865, he was brevetted major general, Volunteers, for gallant and meritorious service during the war.

Roberts continued to serve in the Regular Army, as lieutenant colonel of the 3rd Cavalry, until 1868, then taught military science at Yale University until his retirement on December 15, 1870.