But the jubilant scenes were tarnished as television footage emerged of a man charging into United forward Billy Sharp while he stood on the touchline.
Sheffield United boss Paul Heckingbottom has since revealed Sharp required stitches after being “attacked” as Forest fans spilled onto the pitch.
So what happened?
Here is everything you need to know.
What happened to Billy Sharp?
Sheffield United captain Sharp was sidelined for the game itself because of injury, and was stood on the touchline after Forest secured a dramatic 3-2 penalty shootout victory following a 3-3 aggregate draw, which precipitated a pitch invasion at the City Ground.
Footage shared on social media appears to show one supporter running at the 36-year-old striker and knocking him to the ground with what appears to be a head butt.
The man dashed away from the scene while Sharp lay on the floor.
“It’s assault,” a furious Paul Heckingbottom claimed. “We’ve seen one of our players attacked. He’s shook up, bleeding, angry.
“Bill was minding his own business off the pitch, trying to get our players away. He was blindsided, it was cowardly. He was knocked to the ground (and required) stitches, he was shook up.”
Heckingbottom added on Sky Sports: “We’re seeing things thrown on the pitch, putting players’ health and safety in danger and nothing’s ever done about it.
“How can you come to a place of work minding your own business and be assaulted in that fashion?” he added. “It’s just bang out of order.”
The incident overshadowed Forest’s win, a victory that secured a play-off final against Huddersfield on Sunday 29 May at Wembley, and the chance of a return to the Premier League for the first time since 1999.
The Professional Footballers’ Association has said “matchday security and fan disorder needs to be reviewed ahead of next season”, and that it would “be raising player safety with the clubs, leagues and the UK Football Policing Unit.”
What happens now?
Forest fan Robert Biggs has been jailed for headbutting Sheffield United captain Billy Sharp at Tuesday’s play-off semi-final after downing seven pints.
The Forest season ticket holder, 30, admitted assault occasioning actual bodily harm at the team’s City Ground which left Sharp requiring four stitches to a wound to his lip.
Biggs, a £55,000-a-year electrical engineer, was sentenced to 24 weeks in prison and a 10-year football ban on Thursday (19 May) at Nottingham Magistrates' Court.
Nottingham Forest have announced that the supporter, who has followed the club since he was five years old, will be banned for life.
Biggs drank nearly a gallon of beer before and during the game, and claimed he only realised the degree of the contact after watching video of the incident later, according to Nottinghamshire Police.
A separate charge of illegally entering the playing surface was dropped.
Biggs claimed he and his girlfriend would be unable to keep up monthly mortgage repayments of over £1,000 if he was imprisoned, and District Judge Grace Leong was urged to give him a suspended sentence.
But the judge told Biggs: “Given the gravity of this offence, where thousands were present to witness it, there has to be an element of deterrence.
The judge went on to say that the event appeared to be a "targeted act of aggression" that required a prison sentence.
Biggs, of Church View, Ilkeston, Derbyshire, was ordered to pay Sharp £500 in compensation, as well as £85 in costs and a £128 victim surcharge.
Forest issued a statement apologising to Sharp - who had a loan spell at the club during the 2012–2013 season - and warned the person found responsible will be banned from the club for life.
The club statement read: “Nottingham Forest Football Club are appalled to learn that our former player, Billy Sharp, was assaulted leaving the pitch after tonight’s match at the City Ground.
“The club will work with the authorities to locate the individual in order they are held to account for their actions, which will include a life ban from Nottingham Forest.
“The club would also like to apologise to Billy personally and to Sheffield United Football Club.”
What has Sharp said?
In a statement posted to Twitter, Sharp branded the individual a “scumbag” but said he would not lose his “respect for the Forest fans”.
“One mindless idiot ruined what was an unbelievable night of football,” he said. “Congratulations to Nottingham Forest on their victory and good luck in the final.
“As an ex-Forest player I will not let one scumbag ruin my respect for the Forest fans.
“Incredibly proud to be captain of this group of Sheffield United players, they gave their all and can hold their heads up high. We will be back and go again.
“Thanks for all your messages and support after the incident.”