Maria Stone was sitting at home, in Glebe Avenue, when the singer showed up at her door to tell her she had won.
The 60-year-old grandmother doubled her winnings by buying two £2 tickets for the People’s Postcode Lottery in January.
She said: “I’m overwhelmed. It’s brilliant news.
“I’m so glad I decided to give it a go and that I signed up with two tickets.”
She added: “I work at Pinewood Studios but I’m off sick at the moment.
“I’m really glad of some good news this week as I’ve just been diagnosed with a rare disease called Myasthenia Gravis, which is an autoimmune condition.”
Mrs Stone, who has two children and three grandchildren with her husband Lionel, has already come up with some ways of spending her winnings.
She said: “I can’t really travel at the moment with my illness, but eventually I’d love to be able to visit my sister in the Canary Islands.
“We also need a new sofa, carpet and some bills need to be paid.”
Mr Johnson, who made it to the finals of the sixth X Factor series in 2009, said: “Maria was so stunned with her win, I had to give her a hug.
“She needed some good news this week and I hope she is able to do all the things she wants to with her winnings.”
Mrs Stone was one of two people in her street to win with the postcode UB10 8PF.
The other, who won £25,000 with a single ticket, was not at home when the singer arrived, so the money will be transferred directly into his or her bank account.
Players taking part in the postcode lottery have raised more than £63.6m for various charities and good causes across the UK and internationally.