He received an appointment to the United States Military Academy, becoming a cadet in 1845 and graduated in 1849 as 25th out of 43.

[4] He led a brigade of Virginia cavalry in the Second Battle of Bull Run in August 1862, and the early part of the Maryland Campaign in September 1862.

Prior to the Battle of Antietam, having a very mixed relation with his superiors, he was relieved of his command and ordered to North Carolina to recruit and train new cavalry regiments.

[6] After training and sending north several cavalry units, during the Gettysburg Campaign Robertson was back with the Army of Northern Virginia.

Due to seniority, for parts of the campaign Robertson directed both his and "Grumble" Jones's brigades.

Robertson helped cover Lee's retreat following the Battle of Gettysburg, the brigade having suffered heavy losses during the campaign.