United needed to match Burnley's result to be crowned champions on Saturday due to their superior goal difference and they eventually did just that following an afternoon of toil, beating Plymouth 2-1 courtesy of a last-gasp strike from Manor Solomon.
Daniel Farke's side equalised through Wilfried Gnonto's third effort in as many games shortly after half-time, cancelling out Sam Byram's own goal, before the late drama at Home Park.
After Mihailo Ivanovic's early opener for play-off hopefuls Millwall at Turf Moor, Josh Brownhill took his goal tally for the season to 18 and Jaidon Anthony struck to give Burnley a 3-1 triumph and leave them second with 100 points.
But Blackburn, Millwall and Middlesbrough were left disappointed in their hunt for a top-six spot alongside Sheffield United, Sunderland, Coventry and Bristol City.
Jack Rudoni's brace took Frank Lampard's Coventry to a 2-0 home win over Middlesbrough and Ross McCrorie scored twice as Bristol City fought back to secure a 2-2 draw against relegation-threatened Preston at Ashton Gate.
Blackburn were heading into the top six at the Bristol City's expense when Yuki Ohashi gave Rovers the lead at Bramall Lane, but they were pegged back by Anel Ahmedhodzic's equaliser in a 1-1 draw with Sheffield United.
Luton's second successive relegation was confirmed after a 5-3 defeat at West Brom, with Tom Fellows, Callum Styles and Millenic Alli all scoring twice at the Hawthorns.
The Hatters therefore needed a big favour at Fratton Park, but Hull survived to hang on for a 1-1 draw at Portsmouth, after Christian Saydee equalised for the hosts in the 55th minute.
Derby and Stoke played out a goalless draw to retain their second-tier spots.
Sunderland will go into their play-off semi-final against Coventry on the back of five consecutive losses.
QPR, with assistant coaches Kevin Betsy and Xavi Calm in charge after Marti Cifuentes was placed on gardening leave, claimed a 1-0 victory at the Stadium of Light after Nicolas Madsen's early finish.
