Judith Keppel scooped the biggest prize in British TV history in a nerve-racking contest, filmed on Sunday but to be shown on Monday night.
Now in its eighth series, the show is one of the UK's most popular programmes but until Sunday no-one had walked off with the top prize.
News that someone has finally struck gold it could send ratings soaring.
But it could also create a ratings battle between the BBC and ITV as Millionaire goes head-to-head with the last episode of One Foot in the Grave at 9pm.
Fans of the hit BBC sitcom have been waiting for Monday's programme because it marks the 10th anniversary of the sitcom and shows the demise of favourite TV grump Victor Meldrew.
Millionaire contestant Mrs Keppel is a mother of two from London and is married to a TV scriptwriter.
She is also said to be descended from Alice Keppel, mistress of Edward VII and great-grandmother of Mrs Parker Bowles.
Mrs Keppel told reporters outside her London home that she felt "wonderful" and "rich" after her win.
But she said she was still undecided as to how to spend her money.
"I have not thought about it Christmas is coming," she added.
Neighbour Tony Marziano commented: "It's very good news - let's hope this is a lucky street."
Cliff-hanger
Viewers saw Mrs Keppel win £16,000 in Saturday night's episode.
At the end of the programme she still had two lifelines: 50-50 and Phone A Friend.
She used up her Ask The Audience lifeline on the £16,000 question.
She turned to the people in the studio when stumped by the question: "Prime Minister Tony Blair was born in which country? England, Northern Ireland, Scotland or Wales?".
Fifty-three per cent of the audience voted for Scotland, leaving Mrs Keppel struggling to decide what to do.
She knew that if she answered incorrectly her winnings would drop to £1,000.
She finally opted for the right answer, Scotland
Celador, the makers of the quiz show, have not yet released the full list of 15 questions Mrs Keppel successfully answered.
The show's previous top prize-winner was mother-of-two Kate Heusser who won £500,000 at the beginning of the month.
The finale of One Foot in the Grave also closes a chapter in British TV with the death of one the country's best-loved characters.
At the height of the show's popularity the cantankerous old grouch Victor Meldrew was watched by more than 18m viewers.
On Monday, however, hankies will be at the ready as Victor meets his end when he is run down in the road by a careless woman driver.