Gabriella Cowley dropped three shots in the final three holes and had a four-hour wait before winning the fifth Rose Ladies Series event on Thursday.
The 24-year-old, ranked 576th in the world, was five under after 12 but a double bogey on the 16th and a bogey on the last saw her lead on two under par.
England’s Georgia Hall and Charley Hull both reached four under but had double bogeys as they finished one under.
“It was a nervous wait in the clubhouse but I never lost hope,” said Cowley.
“I had my dad on the bag and it was a brilliant day from which I’ll take a lot of confidence.”
Cowley, from Essex, had not finished outside the top 12 in the first four of the eight-event series, which has been sponsored by major winner and Olympic champion Justin Rose, among others.
And she looked to be setting a tough target at the JCB Golf and Country Club in Staffordshire after a run of three successive birdies from the 10th took her to five under and she backed that up with three pars.
But as Cowley dropped two shots at the 16th to drop to three under, her playing partner Hall was making a birdie to move into the lead on four under.
However, the 2018 Women’s British Open champion fluffed her approach to the last and three-putted for a double bogey to hand the clubhouse lead to Cowley.
They were not the only players to falter in the closing holes. Scotland’s Heather McRae bogeyed the 16th and 18th to finish in the group in joint second on one under.
England’s Annabel Dimmock bogeyed her final two holes to also card a one-under 71, while Hull, who reached four under after 12, had a seven on the par-five 13th and dropped another shot on the 15th.
Wales’ Chloe Williams and England’s Whitney Hillier also shot 71s to finish joint second.