Former Staff Sgt. Tony Davis and Captain Mike Crofts rushed to injured policeman and administered combat first aid, applying dressings and pressure to several of his wounds.
Friends of the former servicemen told the Telegraph they had "fought desperately" to save the married father and were "devastated" by his death.
The two men were leaving a parliamentary meeting on boxing when they saw the suspect stab the policeman before he was shot.
"They tried desperately hard to save him. He was surrounded by friends and colleagues who loved and fought desperately for him; everyone was devastated by the death."
Captain Crofts, 31, is from Cambridge and served twice in Afghanistan with the Royal Tank Regiment and Sgt. Davis, 42, from Darlington, served in Northern Ireland for 5 years and in Bosnia for the PT Corps and Fusiliers.
After running towards the scene the two men were soon joined by a number of police officers and Tobias Ellwood, the Tory foreign minister, who attempted mouth-to-mouth resuscitation.
Mr Ellwood, a former soldier himself, attended the victim as he awaited medical staff and an air ambulance to arrive in Parliament Square.
He was later seen with bloodied hands talking to police officers near the scene before going back to the Foreign Office.
A member of the public is treated by emergency services near Westminster Bridge
Five people were killed in the "sick and depraved" terror attack on Wednesday, while dozens more were injured.
Armed police have carried out an overnight raid on a Birmingham property, which was reportedly linked to the attack in Westminster.
Counter-terror detectives are continuing to search for clues about about how an armed attacker, believed to have "inspired by international terrorism", brought death and destruction to the streets of London.
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