The Staffordshire golfer had gone into the final round of the Canary Islands Championship in the final group after a stunning third round in Tenerife.
But while playing partner Garrick Higgo continued his form by posting a third sub-65 score of the tournament to win by six shots, Mansell finished the day 10 shots back after a one-over-par final round.
The 26-year-old had carded a bogey-free 64 on Saturday to take the outright lead in the European Tour tournament, only for a late surge by South African Higgo to push him back into second.
The former Chase Terrace Technology College pupil, had opened with rounds of 66 and 65 to put him right in contention.
And the rising star, based at Beau Desert Golf Club near Cannock, said he felt he was due a low score at the Golf Costa Adeje course, which also hosted last week’s Tenerife Open.
“I’ve seen going low on this course for two weeks now and I feel like I’ve played a few rounds where I could’ve gone really low and didn’t,” said Mansell. “I missed a few putts early on and I wanted to cut out mistakes and thankfully my short game was good on Saturday and holed some putts, which for the last few weeks has been missing.
“I’ve always said if I can putt pretty good I’m going to be there or thereabouts.
“I just played really solid and the biggest mindset was staying gritty and not letting bad stuff faze me.
“I had a couple of holes where I had a little wobble and short game got me out of trouble.
“At this level that’s what you’ve got to do.”
Higgo, who won the Gran Canaria Lopesan Open a fortnight ago, finished with a winning total of 27 under par – just two shots outside the European Tour record held by fellow South African Ernie Els and meant he was an incredible 68 under overall for the three-week Canary Islands swing.
“I need to check if there’s something in the water,” joked Higgo. “I played so well. I hit the ball very nice so it was a lot less stressful than the last time. I can’t believe it happened so quickly again, but my game has been good so I can believe it as well.
“You definitely dream about all of these things. I definitely believed I could do it, but it is nice that it’s actually happened.”