After both West Brom and Brentford were beaten at the weekend, the Baggies only needed to match the third-placed side's result to seal a return to the Premier League after two seasons away in England's second tier.
A seemingly straightforward home fixture against Queens Park Rangers did not go entirely to plan for West Brom, but a 2-2 draw took it up as Brentford went down 2-1 to Barnsley on Thursday (AEST).
That result saw Barnsley stay up, but Wigan could not join it as it drew 1-1 with Fulham, which could have been promoted with a win, and went down because of a looming 12-point deduction.
Charlton Athletic and Hull City were also demoted, while there was a similarly chaotic finish in the play-off race.
BAGGIES BEST BRENTFORD
Both West Brom and Brentford lost their nerve with two matches of the campaign remaining, but a return of just a single point from a pair of games proved enough for Slaven Bilic's team.
After Saturday's (AEST) defeat to Huddersfield Town, West Brom was boosted by Brentford's own reverse at Stoke City yet could not beat QPR.
West Brom trailed to Ryan Manning's strike before turning the match on its head with goals either side of half-time through Grady Diangana and Callum Robinson, only for Eberechi Eze to equalise just past the hour-mark.
It mattered not, though, as Brentford failed to capitalise, instead losing again.
Josh Dasilva cancelled out Callum Styles's Barnsley opener, but as Brentford pushed for a precious winner, Clarke Oduor tapped in at the other end in the 91st minute.
BRAVE BARNSLEY, WIGAN WOE
There were contrasting scenes at full-time in the final regular league match at Griffin Park after Barnsley celebrated as Brentford collapsed.
🥳 GOING OFF at Griffin Park, but not for @BrentfordFC!! It's @BarnsleyFC doing the celebrating as it pulls off a great escape, a 2-1 win ensuring it remains in the #EFLChampionship for another season 👏🏻👏🏻👏🏻 pic.twitter.com/w3EzLoaU8n— beIN SPORTS (@beINSPORTS_AUS) July 22, 2020
The visitors were heading down until Oduor struck but stayed up at Charlton's expense, the Addicks going down 4-0 at champion Leeds United.
Hull propped up the table after a 3-0 defeat at Cardiff City, while Luton Town stayed up thanks to a rallying 3-2 success at home to Blackburn Rovers, aided by two own goals.
Wigan needed to win because of its deduction after falling into administration, but one of the form teams in the division let a lead slip against Fulham, its total of 59 points bumped down to 47 to fall a painful point short.
SWANS SAVOUR GOAL SWING
With rival Cardiff comfortable in claiming its play-off place, Swansea City needed to beat 10-man Reading while Nottingham Forest lost to Stoke City, along with a five-goal swing.
Improbably, the Swans stole sixth place from Forest after a remarkable finale.
The Welsh side won 4-1, netting its fourth in the 91st minute as Wayne Routledge headed in his second.
That scoreline left Forest – 3-1 down at home to Stoke at the time – needing to score, but instead it conceded, its promotion bid derailing in spectacular fashion as Nuno da Costa stabbed into his own net in the sixth minute of stoppage-time.
The 4-1 defeat meant Forest – without a win since 1 July – finished seventh.
Elsewhere, there were wins for Derby County, Millwall and Middlesbrough, bringing the curtain down on another season in gripping fashion.