Chris Rolfe, 24, saw the vixen being hit by a car as he was driving back from his farm near Cowfold in West Sussex at around 11.45pm.
When he went to check on the fox, who was killed instantly, he noticed her stomach was moving with the unborn cubs inside.
Mr Rolfe said he “instinctively” ran to his car to get his knife, and then performed the procedure.
“I couldn’t think about it too much,” he said. “I just had to perform the C-section because every minute is crucial.”
He added: “It was instinctive, otherwise instead of one life lost, it would have been the death of all of the cubs as well as the mum.”
Mr Rolfe does not have any veterinary training but had performed a Caesarean before during lambing season.
After the procedure, he put the fox cubs into his pocket and took them to his mother, who has experience caring for foxes.
Jean Rolfe and her son immediately cleaned and dried the cubs before putting them them in a cardboard box on top of a heater to keep warm. They then started to feed the cubs puppy milk every 20 minutes.
Ms Rolfe, 51, said: “If Chris hadn’t stopped, they wouldn’t have survived. So Chris just got on with it and dealt with it.
“He was in that frame of mind, I guess because he was looking after the sheep his instinct was there.
“A lot of people talk about having foxes as pets, and even though they are lovely animals, they are wild and they need to be wild.”
Ms Rolfe and her son are working with the Fox Project, a local charity, to ensure the now seven-week-old foxes are healthy and strong enough to be released into the wild when they are six months old.
The cubs have been named Ginger, Little Tip, Big Tip and Biscuit.
Additional reporting by SWNS