Shane Turner, 51, told Malika Bouchiba, 56, she was not entitled to any of the wealth tied up in the home in Twickenham, south-west London.
Although the property was registered in his name, Miss Bouchiba, a parenting coach, had met most of the mortgage payments.
But when they parted, casual worker Mr Turner insisted the mother of their grown-up son was simply a paying guest.
After ruling that Mr Turner’s version of events was “incredible”, Judge Owen Rhys awarded Miss Bouchiba half the value of the house.
He said: “His evidence as a whole contains a series of improbable and implausible explanations which are inconsistent with the documents and other verification produced by him.
“By contrast, Miss Bouchiba was a straightforward and candid witness.
“Her version of the disputed facts was almost always supported by documentation.”
The judge added: “He claims that Miss Bouchiba lived with him as ‘my guest’ and that he paid for the majority of the expenses and living costs and that any payments made by her were reimbursed in cash.
“He continues to live in the house and denies that Miss Bouchiba is entitled to any share of it.
“She says that she was always intended to be a joint purchaser and joint mortgager. This was to be a family home.”
The court heard their relationship began in 1986 and they registered the property in his name because she had a poor credit record.
Mr Turner put up the £40,000 deposit, while she agreed to pay the mortgage and bills, and although only his name was on the deeds, he promised not to “screw her” on the deal.
They have a son – now in his 20s – and while Miss Bouchiba was always in employment, Mr Turner only worked for short periods in part-time jobs and claimed state handouts.
After the split, he refused her demands for half the value of the house and denied that they had been cohabiting. He claimed to have paid back the £17,500 she spent on the mortgage and bills using cash he made from a sideline of selling Koi carp.
In his evidence, Mr Turner claimed his ex-partner was “a bullying and domineering woman” and “a wreckless spendaholic”.
But finding for Miss Bouchiba, Judge Rhys said court papers suggested “a somewhat different character”.
He said: “Her explanation is far more probable – that she was the only one in a financial position to make these payments.”
The judge ordered that the property cannot be sold without Miss Bouchiba being given half the equity and ruled that Mr Turner should…