Leitao's seventh goal of the season, just two minutes from time, made it seven successive home victories for Paul Merson's Walsall side at fortress Bescot.
It was rough on battling Wycombe who had looked set to force the game into extra time when Charlie Griffin's second goal of the game in the 83rd minute had dragged them back on level terms.
But Walsall just held the edge as they survived the shock of falling behind to Griffin's fifth goal of the season to draw level through James Constable's first goal for the club following his recent arrival from Chippenham before Julian Bennett fired them ahead after 22 minutes.
The defeat made it a bad four days for Wycombe who had suffered their first league defeat of the season at Bury last Saturday.
Wycombe made the perfect start when Griffin struck with a smart piece of finishing after just six minutes. He capitalised on the impressive play of Kevin Betsy who made great strides down the right wing and rolled a centre across the face of the penalty area that Griffin drilled home with a low drive.
The shock of falling behind soon sparked Walsall into life and Matty Fryatt and Leitao both went close to drawing them level before Constable capped his first start in style after 17 minutes.
But his goal was a personal disaster for the Wycombe skipper Roger Johnson after he mis-kicked a cross from Craig Pead and set-up Constable who volleyed home from 12 yards.
Walsall were in control at this stage and made that pressure pay when Kris Taylor got the ball into the area from the left-wing at the third attempt with Bennett powering home a decisive close-range header.
Walsall should then have gone on to put the game out of Wycombe's reach at the start of the second half when Fryatt squandered two gilt-edged chances.
They were misses that returned to haunt Walsall seven minutes from time when Griffin pounced.
He took full advantage of a pin-point centre from strike partner Tommy Mooney to force home a far post header despite the best efforts of keeper Joe Murphy.
It looked good enough to take the game into extra-time but Leitao ended their hopes when he started and finished the match-winning move.
He won the ball after a poor clearance from goalkeeper Steve Williams and, after Fryatt and Alex Nicholls combined to set him up, the Portuguese marksman calmly slotted home a low drive.