Taylor Winston, 29, helped dozens of injured people after gunman Stephen Paddock, 64, opened fire from the 32nd floor of the Mandalay Bay Hotel in Las Vegas killing 59 and injuring 530.
As victims ran for cover from a hail of bullets another group of friends set up an area to treat victims who had been wounded by the shooter.
Kacy Thompson told The Daily Beast they used a Sharpie pen to mark victims who needed help in a make-shift triage area they had set up.
He said: “Our triage area was just civilians. I had a firefighter with me, a trauma nurse, and we were going down the line, doing what we could.”
Mr Thomson said they managed to treat at least 40 to 70 people in the triage unit, using medical kits they had taken from police cars.
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As they got to work Mr Winston rushed in to help with his pal Jenn Lewis, using the pick-up truck to load victims in and ferry them to hospital.
Mr Winston managed to gather around 10 to 15 people each time to take them to Desert Springs Hospital Medical Center.
He said: “I luckily found a truck with keys in it and started transporting priority victims to the hospital and tried to help out the best we could until more ambulances could arrive.
“People needed to get out of there, and we tried the best we could to get as many as [we] could.”
Friend, Lauren Wardell wrote on Facebook: “Taylor Winston is a total badass and hero! My roomie saved dozens during the Vegas attack. Pretty amazing what he did.”
The veteran was honourably discharged after serving in the military from 2006 to 2011. He used his vital skills to help others in the chaos that ensued after the shooting.
Meanwhile, a British soldier is also being praised after he too rushed to the aid of the injured.
Ross Woodward, 24, had been enjoying a trip to Las Vegas with other off-duty troops when the carnage unfolded on Sunday night.
Without a second thought, the soldier, from 1st The Queen’s Dragoon Guards, ran to help, along with two other soldiers who he had been travelling with.
His younger brother Curtis Dyer, 22, from Beeston, Nottinghamshire, said his brother was the type of person who would help, “as he is always there to help people when they need his help”.
Mr Paddock caused chaos when he cut short the lives of concert-goers in a frenzied mass murder – the largest in US history.
The 64-year-old gunman had at least 10 suitcases filled with guns when he opened fire from his 32nd floor suite on a country music festival.
Las Vegas police said he had 23 guns at the hotel, including semiautomatic rifles, and 19 at his home along with thousands of rounds of ammunition.
Police are still searching for a motive.