Summer signing Ollie Palmer scored to give Mansfield their first goal and win since returning to the Football League.
The visitors started the better of the two teams and James Jennings went close early on with a well-struck volley.
Mansfield keeper Alan Marriot was forced into action and saved well from a Josh Scowen attempt and Paris Cowan-Hall's long-range effort.
But Palmer won it when he pounced on a rebound after Martin Riley's header hit the post and poked in from close range.
VIEW FROM THE DRESSING ROOM
Wycombe manager Gareth Ainsworth told BBC Three Counties Radio:
"It was unrecognisable from the team we had against Morecambe or Leicester and there's no rhyme or reason to it.
"We were poor today, we were very poor and I won't let that happen again.
"There's not many positives I can take from that match.
"I need to get the belief back in them and get them playing football."