Town looked set to follow up their February win over champions Reading with victory over the Blades thanks to Ahmet Brkovic's eighth goal of the season 18 minutes from time.
But Tonge had other ideas as he drilled home a 22-yard free-kick in the 90th minute after Steve Howard had marred an otherwise faultless display at centre-half by bringing down impressive visiting full-back Chris Armstrong.
Luton produced a tigerish display with a patched up side and even missed a 54th-minute penalty when Rowan Vine blasted over.
Kevin Nicholls, who completed his return from injury by coming on as a second-half substitute, almost secured a win for the Hatters when his 86th minute free-kick hit the top of the crossbar.
But the visitors earned a point with Tonge's late intervention.
With Town suffering a defensive injury crisis, leading goalscorer Howard was forced to start at centre-half for the second home game in succession.
Changes to the side that won 2-1 at Plymouth on Easter Monday saw Vine and Brkovic recalled in place of the injured Markus Heikkinen and David Bell, who dropped to the bench.
Sheffield United made four changes to the side that drew 1-1 with Leeds United in midweek and included former Watford striker Bruce Dyer in their starting 11 up front alongside Ade Akinbiyi, who hit a hat-trick on his last visit to Kenilworth Road in Burnley's 3-2 win in November last year.
The game kicked off in bright sunshine amid a carnival atmosphere as Blades fans celebrated their side's promotion to the Premiership by dancing the conga along the front of the visiting supporters' Oak Road end.
Town followers were also in good voice for the final home fixture of the season and it was Luton who started the brighter as Vine's mazy third-minute run allowed Brkovic to cross to the far post where Dean Morgan controlled on his chest, but saw the ball grabbed off his toes by United keeper Paddy Kenny before he could pull the trigger.
Leon Barnett then caused problems with two headers from set-pieces in the space of five minutes, as he firstly saw Kenny punch clear his effort from Morgan's 15th-minute corner before nodding wide from Peter Holmes' flighted free-kick.
Sheffield United had offered little for much of the game, but they registered the game's first attempt on target in the 34th minute when Akinbiyi nodded on to Armstrong who slipped in Dyer to hit a deflected shot straight at Marlon Beresford from 12 yards.
Two minutes later Dyer threatened again when he seized upon Akinbiyi's knock down and smashed a 25-yard effort just past the post.
Not to be outdone, Enoch Showunmi then repeated the act when he ran on to a loose ball in midfield and made ground before rifling just wide from the same distance.
With both sides seemingly unable to hit the target Carlos Edwards blasted over from 20 yards in the 40th minute after good work from Morgan, just moments before Phil Jagielka hit a speculative effort well wide for the Blades from 25 yards out.
Luton's best effort came in the final minute of the half when a vicious in-swinging free-kick from Morgan found Barnett, whose far-post header was parried by Kenny.
The momentum of the leap saw Town's young centre-half tumble into the moat behind the goal, but he luckily returned to the field unscathed from his fall.
The second half saw the visitors force four corners in the first five minutes, but all they had to show for their possession was a 35-yard Michael Tonge effort that dribbled well wide in the 51st minute.
Instead it was the Hatters who spurned a glorious chance to take the lead when Vine won a dubious looking penalty after falling under Chris Morgan's tackle in the penalty area.
Vine himself took the 54th minute spot-kick, but in keeping with the wayward finishing on show throughout the game, wastefully blasted his effort high over the bar right-footed.
Vine then smashed another effort over just a minute later before Luton finally hit a shot on target in the 57th minute, when Brkovic fed Holmes in the area and the midfielder saw his low, left-foot, 12-yard drive held by Kenny.
The visitors weren't to be left out of the inaccurate efforts on goal when Akinbiyi hit an instant 22-yard volley from Howard's misdirected header in the 63rd minute, but his effort ended nearer the corner flag than the goal.
United were much closer when Akinbiyi and the impressive Armstrong just missed substitute Simon Francis' 69th-minute cross in the six-yard box.
But Town grabbed the goal their play probably just about deserved three minutes later as Brkovic met Keith Keane's deep cross from the left with a majestic header from 10 yards that flew past Kenny.
Fit again Nicholls entered the fray shortly after in place of the hard-working Morgan, to rapturous applause from the home faithful.
And things almost got even better for Town when Kenny pushed aside Warren Feeney's low 22-yard drive in the 77th minute, before Brkovic just missed Showunmi's dangerous left-wing cross three minutes later.
Nicholls then clipped the bar with a free-kick four minutes from time as Luton dominated, but Sheffield United grabbed a share of the spoils with Tonge's late show.