More importantly, the Oakwell outfit reclaimed their Championship status after a two-year absence, with Paul Heckingbottom’s remarkable introduction to managerial life continuing in another dream day at the home of football, 57 days from their JPT success over Oxford UNited.
The Reds fully merited their success, taking the lead after 88 seconds through Ashley Fletcher and a sublime strike from Hammill doubling their tally on 19 minutes.
Millwall reduced the arrears 11 minutes before the interval through Penistone-raised boyhood Barnsley fan Mark Beevers, who watched his hometown club at the 2000 Championship play-off final at Wembley.
But the Reds held their nerve, with Isgrove sealing it amid considerable delight with his first goal in over two years since April 2014 with a header 16 minutes from time.
Millwall suffered a blow in the warm-up with Sherburn-in-Elmet born defender Byron Webster, named in the starting line-up, forced off in the warm-up with a recurrence of hamstring trouble, with Tony Craig stepping into the breach and teenager Jamie Philpot promoted to the bench.
A ripoaring start saw Millwall go close, only for Barnsley to then promptly proceed to go up the other end of the pitch and draw first blood in a truly sensational opening.
First, Penistone-born Mark Beevers headed a good chance wide after leaping highest following Chris Taylor’s free-kick after Josh Scowen untidily felled Shane Ferguson.
But the next action saw the Reds forge in front with Sam Winnall winning the air miles to flick on a ball forward following Adam Davies’s goal kick in the direction of Fletcher, who beat Carlos Edwards before outsmarting the defensive cover and firing an angled low shot past Jordan Archer to the delight of the Reds contingent in just the second minute.
The breathless action continued with Winnall teeing up Hammill, who fired over from a decent position before more hesistancy from the Millwall defence - clearly affected by the late withdrawal of key centre-back Webster - allowed Fletcher to hare clear, but from a narrow angle, his effort was blocked by Archer in a key moment.
Millwall possessed threat going forward themselves in a breathless opening, but the Londoners were then floored by a magic Wembley moment conjured by Hammill.
Finding the ball at his feet on the left on 19 minutes after Conor Hourihane dispossessed Shane Ferugson, Hammill mesmerised veteran Edwards and found a pocket of space after cutting inside before firing home a magnificent right-footed curler from 25 yards out which flew past Archer.
The strike took the wind out of stunned Millwall for a fair spell, but gradually they found their feet and they claimed a lifeline on 34 minutes.
A corner from Ferguson wasn’t dealt with, with Steve Morison heading the ball to the far post.
The danger was only half-cleared before the loose ball found its way to Craig, whose skewed shot found Beevers, who fired home from close range after outmuscling Marc Roberts, a former class mate at Penistone Grammar School.
The Reds were forced on the retreat for the rest of the half with Millwall starting to win their midfield battles and their fair share of second balls, although they suffered a blow on 43 minutes when left-back Joe Martin limped off and was replaced by Aiden O’Brien.
Barnsley started to assert themselves more at the start of the second half with Hammill seeing a shot diverted over by Edwards.
At the other end, a mistake by Scowen almost let in Sheffield-born Lee Gregory, only for Davies to save the day and gather before a neat move at the other end finished with Hammill firing tamely at Archer.
Roberts was booked for a sprawling challenge on Gregory before the Reds were handed a reprieve when Davies was left stranded out of goal with the ball eventually finding its way to O’Brien, whose hooked shot was eventually held by the backtracking Barnsley keeper.
Alert Ferguson defending then denied Isgrove, before the Welsh winger had his moment to savour 16 minutes from time.
He was left totally unmarked from Hourihane’s corner, with Archer and the Lions defence showing a wretched dereliction of duty, with the loanee nipping into head home from close in.
Winnall spurned a great to make it four on 82 minutes, before superb reactions from Davies denied Taylor.
Barnsley: Davies; Scowen, Roberts, Mawson, Williams; Isgrove (Watkins 85), Hourihane, Brownhill, Hammill; Winnall, Fletcher (Toney 81). Substitutes unused: Townsend, Chapman, Nyatanga, McCourt, White.
Millwall: Archer; Edwards (S Williams 82), Craig, Beevers, Martin (O’Brien 43); Taylor, Thompson (Onyedinma 76), Abdou, Ferguson; Morison, Gregory. Substitutes unused: Forde, Philpott,, Upson, Nelson.
Referee: S Attwell (Nuneaton).