Elizabeth Kennet, who is a great-grandmother, confronted the man outside the downstairs lavatory of her home as he tried to escape and kneed him in the groin.
He had been trying to make off with her lap-top computer but dropped everything as he fled the house in Bayswater, London. In his haste he left a bag containing his address-book, a pair of swimming trunks and a towel.
"I think I did exactly the right thing. It certainly upset him," said Lady Kennet. "I have not actually been attacked before but I have always understood that is the thing to do."
Lady Kennet, 72, who is a writer on architecture, arms control and maritime matters, was in the garden with her husband, Lord Kennet, when he saw someone through the bathroom window.
Thinking it was their lodger looking for an aspirin, she went into the house and saw a mousey-haired man aged between about 35 and 40 coming out of her study near her bedroom. "He was carrying my precious lap-top with a lot of work on it and a couple of boxes," she said.
"He had a silver cigarette-case my father had been given as a wedding present. I said: `Who are you?' and he said: `I'm Tom.' I said: `Put those things down' and he said: `I don't want to' and made for the stairs.
"I realised I could have kicked him down the stairs but it would have broken his neck and I didn't think that was quite necessary. He went downstairs and was trying to get out the back but went into the downstairs loo instead.
"I then kneed him in the balls, which upset him quite considerably. I have read that is the thing to do when attacked.
"He was definitely surprised and presumably quite uncomfortable."
She added: "He then did find the back door and ran out, carrying everything. I called out to some passers-by, who asked if they were my things. I said: `Yes, he's a burglar'."
The intruder dropped everything as he fled, fortunately not breaking the lap-top, and vanished down Lancaster Gate.
Lady Kennet said her family had nicknamed her a "have-a-go great granny".
All in all, it was a very eventful day in the Kennet household, for that night her granddaughter gave birth to a son.
Lady Kennet said that she was "extremely clear-headed" when the incident took place and fear did not come into it. "It seemed to me that if one is in this sort of situation, there are no external thoughts at all except: `This chap needs to be stopped'."
Police have the burglar's address book and are investigating the incident, which took place last week.