Chris Porter's header six minutes from the end was enough for Danny Wilson's side, who put to rest the Hertfordshire club's hopes of a record three successive promotions.
The Blades will now face Huddersfield Town or MK Dons at Wembley Stadium for a place in the Championship.
Patrick Agyemang was presented with an opportunity inside the first minute for Stevenage, but the striker saw his shot blocked for a corner, which the home side eventually cleared.
And the hosts were nearly in front just a few minutes later as Lee Williamson chested a cross into the path of Porter, whose header from close range was tipped wide by Chris Day.
Wilson’s side dominated the opening 30 minutes without creating any further clear-cut chances as Stevenage struggled to keep possession of the ball for an extended period.
Williamson tried his luck from long range but blasted over as the half drew to a subdued close, with Kevin McDonald receiving treatment after turning his ankle.
Sheffield United started the second half brightly, going close on two occasions as Ryan Flynn’s shot was kept out by Lawrie Wilson, and Williamson failed to convert a Stephen Quinn, pulling the ball narrowly wide.
Flynn was involved again as he picked up the ball 20 yards from goal and unleashed a drive which again sailed wide of the goal.
|41.0||MK Dons are 41.0 with 188bet to beat Huddersfield 3-0 on Tuesday.|
In an attempt to get a foothold on the game, Stevenage boss Gary Smith removed Robin Shroot for John Mousinho, with the hosts fully in control.
And the visitors were finally able to carve out an opportunity as Scott Laird played in Luke Freeman, but the winger’s effort was comfortable for Steve Simonsen.
Smith was forced into another change with less than half an hour remaining, as Chris Beardsley replaced the injured Agyemang in attack, but the hosts continued to press for a goal as the game approached extra time.
But Stevenage nearly went ahead with just over ten minutes to play when the ball fell to Laird, but his swerving volley was pushed over the bar by Simonsen.
Bostwick was the next to test Simonsen, who was equal to the midfielder’s long range free kick, while at the other end Matt Lowton saw a shot blocked, before left-back Matt Hill’s stunning long distance thunderbolt crept just wide of the angle of post and bar.
With six minutes remaining, Sheffield United finally broke the deadlock as Lowton drifted a cross into the Stevenage penalty area and Porter rose highest to nod past Day.
As a final roll of the dice, Stevenage replaced midfielder Joel Byrom with striker Craig Reid but they couldn't find a goal and will be plying their trade in the third tier again next season.