The ex-Pompey stopper used his experience of such occasions and palmed away two spot-kicks after AFC Totton held AFC Bournemouth to a 1-1 draw after extra time.
In a tight contest, the Stags opened the scoring ten minutes from normal time when Luke Roberts capitalised on a defensive mix up, slotting home from close range.
The Cherries, made up largely of development squad players, showed great character to respond and level two minutes later when Aristide Bassele rose highest to head Harrison Gilkes' free kick into the net.
After a tense period of extra time, the Cherries held their nerve to convert each of their penalties and record a 4-2 win in the shoot-out.
With many of Chris Hargreaves' development squad players out on loan, the 40-year-old handed Flahavan and Charlie Sheringham their first competitive starts since sustaining their respective injuries.
For Sheringham, it was his first appearance since Boxing Day 2011, whilst Flahavan has been out since August 2012.
Another noticeable inclusion was that of Baily Cargill, who started for the Cherries despite currently being on a work experience loan deal with the Stags.
AFC Totton, who have themselves reached the final of the competition for the past three years, included four ex-Cherries in their starting eleven - James Coutts, Carl Pettefer, Matt Oldring and Jamie Whisken.
The Cherries started the game the stronger of the two sides, with Josh Wakefield the first to try his luck, curling an effort wide from the edge of the area.
As the game progressed, the Stags found their rhythm. Following a foul from Josh Carmichael, the home side forced a double block after pumping the ball into the box from the resultant free kick.
A minute later and Carmichael burst through at the other end and took a shot from the edge of the area. His drive had the home fans’ hearts in their mouths momentarily, but his low drilled shot cannoned wide and off the back board.
Totton had the better of the remainder of the half, with Coutts shooting wide from a corner on 38 minutes and Gillespie forcing Flahavan to hold well a minute later.
On the stroke of half time, Gillespie again shot wide under pressure - with Flahavan clearly worried as he scrambled across his goal line.
The influential Carmichael looked to have sustained a knock late in the first period and was replaced alongside Sheringham at the break, with Alex Moth and Jamal Bajandouh entering the fray. And in comparison to a fairly lacklustre first half showing, the game livened up upon the restart.
Bassele, who had looked lively all evening, was put through one on one with the Stags' keeper. With two defenders hot on his heels, he couldn’t keep his shot down and it went over Ross Casey’s bar.
The winger then had another chance just a minute later, but his curling shot from inside the area was blocked and then cleared.
Both Bassele, who was increasingly becoming the Cherries main goal threat, and defender Case, came close for the visitors but it was the hosts who looked the more likely to break the deadlock.
In the 70th minute, Whisken’s glancing header went wide of Flahavan’s post. Urged on by the increasingly lively crowd, he then rose well again seven minutes later but headed Coutts' free kick wide.
However, in reward for their possession and creativity, the Stags finally got their goal.
On the 80th minute Luke Whitley floated a cross into the area, and misjudging the flight of the ball, Flahavan came out to meet it but only saw it go beyond him, where Roberts was on hand to tap it into the net.
The Cherries refused to let their heads drop, and immediately went in search of an equaliser.
Gilkes was felled 30 yards from goal and elected to cross the free kick into the crowded penalty area. Bassele met the ball 15 yards out and cushioned his header perfectly over the rooted Casey, to level the score with just seven minutes remaining.
The goal sparked life into the Cherries, but with the scores still locked 1-1 after 90 minutes, the tie went into extra time.
A nervy thirty minutes ensued, with neither team wanting to throw the game away.
Ultimately it was little surprise when the referee brought 120 minutes of entertaining football to a close, and sent the Cherries to their second consecutive penalty shootout in the Hampshire Senior Cup.
It was Totton who took the first, and immediately Flahavan announced his return to action by stretching to palm it wide.
From then on momentum was with the Cherries and Cargill cooly converted against his loan club to give the side an early lead.
After conversions from both sides, Flahavan got his body behind the ball once more - completing a second impressive penalty stop and teeing up Josh Wakefield, and finally Gilkes to send the Cherries into the final 4-2 on penalties.
AFC Totton: Casey, Oldring, Whitley, Bottomley, Whisken, Mason, Pettefer, Roberts, Coutts, Gillespie, Rudo.
Subs: Odam, Scott, White, Mouland, Goodger.
Goals: Roberts 80'
Cherries: Flahavan, Blackmore, Case, Turner, Cargill, Wakefield, Bassele, Carmichael, Goodship, Gilkes, Sheringham.
Subs: Moth, Bajandouh, Kaye, Harris, Tierney.
Goals: Bassele 83'