The striker scored five minutes from time against Stevenage to book a Wembley date with Huddersfield Town or Milton Keynes Dons on May 26.
Ignoring pleas from the PA announcer, home supporters stormed the pitch at the final whistle and mobbed their own players.
It was hardly the best idea – especially if one had been injured ahead of the next important game.
It was also evident that some were intent on inciting the away supporters when they surged to the opposite end of the ground.
Stewards and police kept things under control but the situation could easily have spilled out of control and taken the gloss off a major victory.
The Blades finally re-emerged from the dressing room 15 minutes later and embarked on a deserved lap of honour.
They have been one of the best three teams in League One this season and, arguably, might have gone up automatically but for the imprisonment of striker Ched Evans with three games to go
The Blades have won nothing yet, of course, but are desperate to join neighbours Sheffield Wednesday in securing promotion to the Championship.
Having held Stevenage to a goalless draw three nights earlier in the first leg, they had to wait until five minutes from time before Porter secured the victory.
Amazingly, Porter was the only senior striker still standing at Bramall Lane after Richard Cresswell was ruled out before kick-off with an eye infection.
Cresswell, who sat on the bench for the first leg protecting a shoulder injury, was expected to start last night.
His absence prompted manager Danny Wilson to stick with an unchanged side when, ideally, he might have preferred to bolster his attack in the home leg.
The Blades had lost three other strikers in the last few weeks – all for different reasons (Ched Evans, James Beattie, and Will Hoskins) – and the best remaining option was midfielder Stephen Quinn in a supporting role to Porter.
The Blades had played their last two league games in a new ‘retro style’ home shirt – a move aimed at boosting sales over the summer months.
Whether a coincidence or not, it had failed to inspire a victory – so the return of the shirts which accompanied better results this season should have been a good omen last night.
The Blades made a nervy start, conceding a corner after just 28 seconds, but the first half turned into a tedious affair with few goalscoring opportunities.
Although Stevenage goalkeeper Chris Day dived full length to keep out a header from Porter after seven minutes, it would be the only close shave before the break.
There was nothing to choose between both sides and, from an early stage, it looked like penalties might well be needed to settle the contest.
Quinn was an industrious protagonist but spent the majority of his time dropping into a five-man midfield with captain Michael Doyle – who oozed determination – and Kevin McDonald, who the Blades were relying heavily on to get the game moving.
McDonald suffered a knock just before break and had to be replaced by Nick Montgomery who had not figured since early March.
The Blades, who were now attacking the Kop, could have scored at the start of the second half but Ryan Flynn’s close-range shot was blocked in the goalmouth by Laurie Wilson.
Stevenage began to come under pressure and Lee Williamson turned a cross from Porter narrowly wide before Flynn hit a 25-yard drive inches the wrong side of a post.
Former Blades defender Kyle Walker had stood among the away supporters at the Lamex Stadium on Friday night and was again in attendance last night.
This time, the England international got an escort around the pitch to the corporate areas and was carrying a limp after picking up a knock in Tottenham’s final Premier League game of the season.
Stevenage manager Gary Smith sent on striker John Mousinho and the balance of play briefly shifted.
Dangerman Luke Freeman skipped past Matt Lowton in the area but his low drive was saved by Blades goalkeeper Steve Simonsen. Tackles became more robust as the tension grew. Both sides were picking up injuries and Patrick Agyemang limped off for Stevenage.
There were ironic cheers from the Blades fans when referee Nigel Miller awarded a free-kick after ignoring a string of other appeals. But when Williamson duly wasted the deadball, fans in the away end replicated the gesture.
Stevenage remained a threat on the counter attack and defender Scott Laird brought a finger-tip save from Simonsen.
But the Blades pumped up the pressure in the final stages and Stevenage’s defence finally cracked with five minutes to go.
Lowton’s cross found Porter who rose highest to head home from close range. It was his eighth goal since arriving from Derby County last summer and easily his most important.
Sheffield United: Simonsen; Lowton, Maguire, Collins, Hill; Williamson, McDonald (Montgomery 45), Doyle, Flynn; Quinn; Porter. Unused substitutes: Howard, Taylor, McAllister, O’Halloran.
Stevenage: Day; Lascelles, Ashton, Roberts, Laird; Bostwick, Wilson, Byrom (Reid 87), Freeman; Shroot (Mousinho 57), Agyemang (Beardsley 63). Unused substitutes: Julian, Charles.
Referee: N Miller (Durham).