Southampton made it 19 Championship matches without defeat as they finished the season with a 3-1 win against Preston at Deepdale.
Taylor Harwood-Bellis' superb 12th-minute header opened the scoring and Ross Stewart doubled the lead with a well-taken second after 47 minutes.
Lewis Dobbin capitalised on a Daniel Peretz howler to halve the arrears on the hour but substitute Cyle Larin tapped home to seal the win in stoppage time.
Tonda Eckert's men may have missed out on the automatic promotion spots but they have completed a Lazarus-like recovery since their early-season struggles under Will Still and will be feeling confident facing Middlesbrough in the play-offs.
Southampton are one of the division's in-form teams and nearly took an early lead when Tom Fellows found Ross Stewart with a delicious cross but the former Sunderland man headed over when he should have hit the target.
It was little surprise when they did go ahead with Harwood-Bellis emphatically heading Wellington's floated cross away from Daniel Iversen and into the corner from close to the penalty spot.
While the Preston goalkeeper had little chance with the goal, he nearly had egg on his face less than 60 seconds later after slamming his clearance straight into Stewart with the ball looping agonisingly wide of his vacated goal.
The Saints started confidently but Peretz had to earn his money shortly after the goal with a remarkable double save, parrying Alfie Devine's half-volley from the edge of the box before reacting brilliantly to keep Ben Whiteman out.
Many of the best chances were coming from the air with Liam Lindsay heading wide from close range, unmarked from Whiteman's corner, just after the half-hour mark and Dobbin heading straight at Peretz having latched onto Andrija Vukcevic's teasing delivery.
David Cornell replaced Iversen at the interval and his first action was to pick the ball out of the net as Stewart snuck a shot into the bottom corner having been given an age to receive Cameron Archer's pass and find an angle with Lindsay unable to block.
An incredible set of events in the 60th minute brought Preston back into the tie as Cornell, first of all, dived low to keep out Shea Charles from the edge of the box before clattering the ball forward towards Dobbin, who chased in hope rather than expectation, only for Peretz to slip as he came out of his goal to sweep, gifting the on-loan Aston Villa man an empty net to halve the deficit.
The home fans were buoyant and North End gave a good account of themselves but they shipped a 92nd-minute third as Larin knocked fellow substitute Samuel Edozie's wicked cross into the net from point-blank range.
Play-off semi-final first legs
Play-off semi-final second legs
Play-off final
Preston's Paul Heckingbottom:
"I couldn't have asked for much more really. We were up against the best team in the Championship at the minute.
"I didn't want us to sit off them and show them too much respect and I felt we were the better team in the first half. They scored that second goal at a killer moment at the start of the second half but we showed good character to get back in it.
"The third goal for them flattered them and I didn't think we deserved it having got back in the game so I'm proud of the effort.
"It's probably a really good reflection of the season of us giving our all, which has been more than good enough against a lot of teams, but we came up against a team today who were better than us."
Southampton's Tonda Eckert:
"No one cares in the end if we make it all the way and you don't make the last step. We need to make the last step.
"The (players) are playing at their best that they have played all season so we are OK.
"It is very important (to keep up standards). It's not easy to play a game like we did today, especially if you have seen our schedule. The performance was excellent in every regard possible.
"The boys were fantastic today. I think that we can suffer in moments and it sometimes helps in away games that you can go a little bit deeper and then break into space.
"I think you can rate that performance at the end of today."
