Alan Dunne saved Millwall's blushes as The Lions booked a place in the Carling Cup second round with a hard-fought victory at Adams Park.
Neil Harris fired Kenny Jackett's men into a 25th-minute lead as The Lions took control during the first half.
But the visitors were in danger of being the architects of their own downfall as they took their foot off the gas after the break and let Wycombe right back in, culminating in sub Ben Strevens striking the leveller 16 minutes from time.
Thankfully it was last year's Player of the Season Dunne, a 70th-minute replacement for Marc Laird, who swept the ball home in the 15th minute of extra-time to end League Two Wycombe's brave resistance.
Lions boss Jackett made four changes to the team that brushed aside Bristol City on Saturday with Harris one of four players drafted into the starting line-up. Tamika Mkandawire and Laird formed a new central midfield partnership in the absence of Liam Trotter and Darren Carter, while Jack Smith got an outing at right-back in place of Dunne.
Millwall's first noteworthy attack came on nine minutes; Laird switched play to Scott Barron and a good first touch by the left-back enabled him to deliver a ball that was cleared for a corner.
James Henry took the resulting setplay and when his first cross was cleared, the return picked out Darren Ward on the edge of the area, but the defender's left-foot bobbler bounced well wide of the mark.
The Lions were denied a 12th-minute opener when Smith's crisp drive smacked the foot of the post with keeper Nikki Bull stranded.
Bull was then called into action, making a smart stop to block Kevin Lisbie's 15th-minute strike as The Lions continued to press.
Lisbie, who twice hit the woodwork at Bristol City on Saturday, was denied his first goal in a Millwall shirt a couple of minutes later with a smart snap-shot that crashed off the inside of the right hand post and away to safety.
At this stage the pressure from Millwall was relentless and the opening goal duly arrived midway through the half. Schofield did well to create an opening and although his angled drive was parried by Bull, there was Harris to calmly slot home the loose ball from six yards.
It could have been 2-0 just moments later when Harris set up Lisbie, but this time keeper Bull charged to the rescue with a brave low save at the striker's feet.
It took Wycombe until eight minutes before the break to muster the first of their two shots on target in the opening half, a low drive from Matt Phillips that David Forde saved comfortably.
Forde was equally well-placed to deal with Kevin Betsy's strike as Gary Waddock's side failed to really pose much of an attacking threat.
A half-time gee-up from Waddock, however, had the desired effect and after the break Wycombe committed more men forward, playing with greater urgency and energy.
The travelling support breathed a collective sigh of relief as Betsy's fierce drive flashed a foot wide.
Danny Foster's first-time curler was held by Forde, and an almighty scramble eventually came to nothing as the home side took the initiative.
As the half wore on, so Waddock introduced his subs and The Lions, who had looked to be comfortable absorbing the pressure, suddenly found themselves staring at a potential defeat when Strevens thumped home the equaliser after good work by Phillips on the right.
The shock of that goal finally saw Millwall spring into action and although Harris' shot was pushed round the post for a corner and then Schofield's curler was clawed away, the visitors were unable to muster a second goal during the 90 minutes.
Extra time called for fresh legs and Jackett introduced John Marquis for the hard-working Lisbie on 98 minutes.
Marquis could have scored with virtually his first touch, latching onto an awful throw by Bull, only to fire straight at the keeper.
But it was Dunne who finally scored what proved to be the decisive goal with a clinical finish following great work and an intelligent pull-back from the byline from fellow sub Marquis.
Two further long-range efforts from Dunne went close, whilst Schofield's goalbound shot looked to be handled by Alan Bennett as Millwall pressed forward and finished in control of proceedings.

Wycombe Wanderers: Bull, McCoy, Foster (Sandell, 61mins), Bennett, Johnson, Phillips, Montrose, Murtagh (Bloomfield, 64mins), Betsy, Rendell (Strevens, 70mins), Pittman. Subs not used: Arnold, Westwood, Westlake, McClure.
Scorer: Strevens (74mins)

Millwall: Forde, Smith, Barron, Mkandawire, Robinson, Ward, Schofield, Laird (Dunne, 70mins), Harris, Lisbie (Marquis, 98mins), Henry (Hackett, 106mins). Subs not used: Mildenhall, Frampton, Grabban, Grimes.
Booking: Mkandawire (44mins)
Scorers: Harris (25mins), Dunne (104mins)

Ref: Trevor Kettle
Att: 3,028