Along with his cousin, Richard B. Garnett, Robert attended the United States Military Academy in West Point, New York, graduating 27th in a class of 52.

In 1843 Garnett became an assistant tactics instructor at West Point before becoming an army recruiter and then an Aide-de-camp to General John E. Wool.

The much traveled Garnett was promoted to major of the 9th U.S. Infantry and went west to the Washington Territory, where he served in the 1856 Yakima Expedition and the 1858 fighting against the Puget Sound Indians.

He requested and was granted an extended leave of absence later that year, when his wife and young son died from disease and he returned east to bury their remains.

He deployed his forces at strategic points along the Staunton-Parkersburg Turnpike, hoping to defend the vital supply route against Federal troops.

After a defeat at Battle of Rich Mountain, Garnett withdrew from his Laurel Hill entrenchments under cover of darkness, hoping to escape to northern Virginia with his 4,500 men.

While directing his rear guard in a delaying action at Corrick's Ford, Garnett was shot and killed during a Union volley.

In 2017, after the white supremacist rally in Charlottesville, Virginia, the city of Monterey installed a new plaque omitting mention of Garnett's Confederate history; it was stolen in 2020.