The towering marksman, who was picked up after being released by non-league Weymouth three weeks ago, latched onto a flick-on by Lee Bell and, after shrugging off the challenge of David McCracken, he coolly tucked his low shot under Wycombe stopper Frank Fielding.
Mansfield, who have now won their last four games on the road, stunned the hosts in the eighth minute when a Michael Boulding flick-on found Louis in space.
His run into the box was crudely stopped by a mis-timed challenge from Gary Holt. Roger East had no hesitation in pointing to the spot and Boulding subsequently rattled home his 19th goal of the season.
Wycombe responded well and were almost level in the 25th minute.
Russell Martin's deep cross found Craig Woodman in space in the Mansfield box but his shot was fired narrowly over.
The visitors then survived another scare after neat work by Sergio Torres set up Martin but his curling shot was inches too high.
Wycombe's domination of possession eventually paid off three minutes before half-time and once again it came from a familiar name.
A lovely back-heel by Torres put Woodman away down the Wycombe left and his cross was expertly headed home via the underside of the bar by Scott McGleish for his 19th goal of the campaign.
The second half continued in similar fashion with Wycombe dominating the game and Torres was unlucky to see his shot creep wide after a neat interchange with Leon Johnson.
The hosts then went agonisingly close to taking the lead, John Sutton's powerful header from Martin's cross crashing back off Carl Muggleton's crossbar.
And Wycombe were made to pay for the miss when Louis struck on the stroke of full-time.