York-based army officer Lieutenant Jared Bambridge, 23, had been to see Romeo and Juliet at the Globe Theatre when the horror unfolded around him on Saturday night.
The off-duty soldier used his training tor treating battlefield wounds to give first aid to those who had been stabbed.
Bambridge, who serves with the Yorkshire Regiment, tended to a 51-year-old man who had been knifed twice, and eventually helped 10 injured people over two hours while police hunted the three attackers.
He and his girlfriend heard gunshots after leaving the theatre, where they had been to see the Shakespeare play for a birthday outing.
He told army media channel Forces News:
"The thing that helped me out most was the ability to remain calm in the scenario. There were lots of people panicking."
He rushed in and out of the inner police cordon to assist medics, including a woman with a large stomach wound whom he performed CPR on for around 30 minutes.
The officer only passed out of training at the Royal Military Academy, Sandhurst last April, and joined his regiment in November.
He returned to his battalion on Monday.