Twenty-six-year-old checkouts colleague Tarah, who is a trained first aider, even leapt over a wall to get to the man, who'd stopped breathing and was being given CPR by an off-duty nurse.
Tarah, who won medals in the 200 metres while growing up in Southend-on-Sea, said: "I was just at the kiosk in the store when a man ran in and told me what had happened. I just shouted 'first aid', grabbed the defibrillator and ran out. I wasn't going to wait around.
"Luckily for me I used to be a sprinter when I was growing up through school – I used to be the one to beat.
502 Bad Gateway
!I just ran as fast as I could and at one point jumped over a wall, which was a bit higher than I thought it was.
"The man had had a cardiac arrest at the wheel and another person in the car had managed to get him out of the vehicle and laid him in the road.
"When I got there, there was already an out-of-hours nurse doing CPR on him who was very well trained.
She used the defibrillator on him which tided things over until the ambulance crew arrived and he was taken to hospital.
The quick-thinking life saver added: "I'm proud of what I did, but I just did what I had to do. I couldn't mess about; it was a life and death situation.
"Hopefully the gentleman got the help he needed in hospital, bless him."
Store manager Tony Blake has nominated Tarah for an Asda service superstar award for her actions.
He said Tarah was "truly, inspiring" and that the whole store was very proud of her.
He said: "Doing what she did meant the man was able to be helped until paramedics arrived and stabilised enough to be moved.
"Her actions made a big difference. It really was going above and beyond."