The Prince, 33, served as part of the Army's Household Cavalry's Blues and Royals, formerly known as the Royal Horse Guards and the Royal Dragoons until 2015.
Training at Sandhurst before being deployed to Afghanistan, he was made a captain and regularly sports his old uniform on royal visits.
Harry never grew a beard as a soldier in line with the Army's ban on facial hair, but grew one again a few months after he left the forces.
He said: 'He's not able to wear his Royal Horse Guards uniform with a beard.
'He does it sometimes but they don't like it. That's why when he took the parade at Sandhurst, he wore a suit.'
Despite the military's beard ban, Harry is no longer a serving officer and is therefore not subject to their rules. But in line with convention, there is a chance he might shave it off.