Chris Wilder's Blades secured promotion back to the Championship on a sunny afternoon in Northampton.
A full away allocation of 1,456, plus more than 6,000 watching the game, beamed back at Bramall Lane, celebrated after the final whistle.
Fleetwood's defeat at Oldham and second half goals from Leon Clarke and John Fleck at Sixfields means the Blades cannot finish outside of the top two.
Boss Wilder made changes to his forward line for the game at Sixfields. Carruthers, O'Shea and Clarke all started - Duffy, Hanson and Sharp dropped to the bench, and were joined by Ebanks-Landell. Coutts took the skipper's armband.
United, with Clarke up top on his own supported by Carruthers and O'Shea, made a bright start with three good chances in the opening ten minutes. Carruthers began a move that ended with him hitting a half-volley against the Northampton crossbar and then O'Shea curled wide and Coutts fired over with an acrobatic effort.
Northampton's first opportunity saw Richards volley over from a central position just six yards out and then Anderson, from distance, was off target. Just before the half hour the home side went close again when Smith raced onto Taylor's through ball but Moore was quickly down to make a one-handed save.
As the break approached, the chances slowed. Fleck tried to catch out goalkeeper Smith with a free-kick but in the one and only minute of added time, Northampton grabbed a half-time lead.
United were on the attack but Clarke's back post cross was mopped up and a break down the right saw Smith lay off to RICHARDS, who, from an acute angle, blasted into the top corner.
Sharp replaced Carruthers at the start of the second half as United changed the approach. A Fleck corner on 50 minutes saw O'Connell show some neat touches but his shot was kept out by goalkeeper Smith.
Hanson was introduced for O'Shea and the Blades were now getting a firm foothold - emphasised just after the hour by the equaliser. Fleck fed the ball through the defence and CLARKE got around the back, just managing to stay onside, before firing high past Smith. Cue celebrations, part one.
From here on in, it was one-way traffic with the Blades pushing for a winner and Cobblers goalkeeper Smith in inspired form, he flew across his goal to keep out a Basham header from a Coutts corner.
Smith then kept out a deflected Sharp shot following Fleck's pass and then Clarke headed over a deep Lafferty cross.
As the game entered the final 15 minutes, a corner routine saw Fleck beat his marker but Smith kicked away and Wilder made a late change with Ebanks-Landell taking over from Clarke.
But there was further drama. On 88 minutes, Sharp went down the right channel and pulled back for FLECK, who emerged with the ball despite two Northampton defenders in the vicinity, before poking past the exposed Smith. Cue celebrations, part two.
After the restart, five minutes of added time passed without further incident but with everyone associated with Sheffield United just waiting to start the party. The referee's final whistle was that sign. Cue celebrations, part three.
Northampton: Smith, Moloney, Buchanan, P. Anderson (McWilliams 70), Diamond, Richards, O'Toole, Nyatanga, Smith (Revell 70), Taylor, Williams (McDonald 81).
Unused: K. Anderson, McCourt, Phillips, Cornell.
Blades: Moore, Fleck, O'Connell, Basham, Wright, Coutts, Freeman, Lafferty, O'Shea (Hanson 53), Clarke (Ebanks-Landell 82), Carruthers (Sharp 45).
Unused: Long, Done, Duffy, Chapman.