Percival attended some of the finest military academies in England as a young man to follow his family's expectations that he become a fine military officer. However, he became bored of the military and spent his time womanising and partying. He fell behind in his studies and was eventually asked for his resignation. His older brother remained at the academy, but Percival left his family, ashamed to face them again. When World War II broke out, Percival enlisted and became a commando at the rank of private.
Pervical was sent to the Howling Commandos as a replacement for their deceased member, Jonathan "Junior" Juniper. Although they were initially skeptical if Percy had the makings of a Howler, he quickly proved himself on their mission to stop Baron Heinrich Zemo who had created a death-ray. Percy continued to serve the Howlers with distinction for the remainder of the war.
Later, the Howlers (Howling Commandos) were called back to service in the Korean War, Pinkerton was a sergeant as were all the other former private soldiers of the unit. At the time of their 1966 Vietnam War mission Pinkerton was managing a Playboy Club in London.
Like several other of the Howling Commandos, Pinkerton briefly joined SHIELD as a favour to Nick Fury after the attack of the Deltites.
He died of cancer in his bed, with no family around but with Dum-Dum Dugan holding his hand while he took his last breath, dying content.
Pinkerton wore a Carlist Requetés type red beret with a yellow tassel with his Battle Dress that may have involved service in the Spanish Civil War. Pinkerton also wore his battle dress with an open collar with scarf rather than having the buttoned collar of a British other rank.
Pinkerton wielded a rifle and grenades in combat. His chief weapon, however, was his bumbershoot (umbrella). He used this device as a club, fenced with it as a sword, used it to aid him in climbing, to slow his descent while falling, and to shield himself from sunlight.
Stan Lee stated that Percival is gay in an interview with On The Media.  However, this is contradicted by Percival's backstory, written by Lee himself, where he was portrayed as a "skirt-chaser". 
His biggest fear was dying alone, but he never feared to put himself in situations that could make that fear come true. At his last hour, he died of cancer in his bed with no family around but with Dum-Dum Dugan holding his hand.