Porter had not scored for the home side since January, but settled the match on 85 minutes when he arrived to meet Matthew Lowton's cross to score from close range. It was the first time he had found the net in 17 games.
Stevenage huffed and puffed without ever doing much in attack as large swathes of the match resembled Friday's 0-0 draw in the first leg.
United dominated possession before eventually finding a cutting edge to cut open the visitors, who won promotion to League One through the play-offs a year ago. United will face Huddersfield or MK Dons in the play-off final at Wembley on May 26.
They play to a finish on Tuesday night with Huddersfield holding a 2-0 lead from Saturday's first leg before they host Karl Robinson's MK.
The first half was almost a non-event in terms of chances with visiting goalkeeper Chris Day nudging the ball wide from a Porter header on six minutes.
United began to exert themselves after the break with Lawrie Wilson blocking a shot by Ryan Flynn from 12 yards out on 48 minutes.
Lee Williamson then slipped an effort narrowly wide on 54 minutes from Lowton's cross before Flynn cut a fine effort wide of the target from long range.
Stevenage finally managed to get away their first shot at the home goal on 58 minutes as Steve Simonsen was forced to dive to keep out a smart effort by Freeman.
With extra-time looking increasingly likely, Stevenage began to come out of their shells as Simonsen tipped a rising Scott Laird effort over his bar before lowering himself to hold Michael Bostwick's driven free-kick.
Hill left Day with no chance on 80 minutes when he saw a howitzer whizz inches wide from 35 yards out before Porter had the final say five minutes later. It was about the right outcome on the night from a largely forgettable contest.
Sheffield United have lost in five previous play-off campaigns, but will hope to win their fourth play-off final.