The Argentinian turned home Garry Kenneth's powerful volley with his hand to help edge United past Partick Thistle and into the Active Nation Scottish Cup fifth round.
The Irn-Bru First Division side gave as good as they got and Jags substitute Liam Buchanan missed two great chances before David Goodwillie netted in the last minute.
But it was Casalinuovo and referee Dougie McDonald who remained the villains for the Firhill faithful. Thistle goalkeeper Jonny Tuffey reacted just like England's Peter Shilton against Argentina in the 1986 World Cup, racing off his line to confront the referee and his assistant after the 26th-minute goal.
Television pictures vindicated Thistle's anger as they showed Casalinuovo had used his hand to direct the ball home from close range after Kenneth's shot had zipped beyond his stooped head. The controversial victory was caretaker boss Peter Houston's first in four matches and lifted United following their failure to strike a compensation deal with Bohemians for their manager, Pat Fenlon.
United started brightly and Craig Conway got an early shot on target before Tuffey reacted well to prevent former United defender Alan Archibald putting through his own net.
Slovakian goalkeeper Dusan Pernis made an assured United debut, kicking and handling well, but he should have been worked harder when Ian Maxwell headed straight at him after John Robertson had headed on a Mark Corcoran corner.
Thistle were finding room down the left flank and Simon Donnelly missed a glorious chance in the 16th minute when he headed over unchallenged from six yards after an Archibald cross.
The visitors missed an equally good chance moments later when Tuffey got down to push away Casalinuovo's low effort from the edge of the box after the forward had shrugged off a defender to run on to David Robertson's chipped pass. Casalinuovo made amends by dubious means after a long free-kick into the box. Thistle bristled with a sense of injustice and Maxwell volleyed well over from 16 yards before Pernis saved comfortably when Steve Lovell found space inside the box.
But Tuffey kept them in the game when he parried Conway's shot from the left channel as the unmarked Jon Daly awaited a square ball.
The action continued and Casalinuovo glanced a header beyond the far post from Dixon's cross before Donnelly came close from 25 yards.
Tuffey made another good save in the closing stages of the half, turning Mihael Kovacevic's drive behind his near past.
United comfortably survived some early second-half pressure with Corcoran and John Robertson failing to trouble Pernis from the edge of the box and Casalinuovo shot into the side net as the visitors began to control possession.
But Buchanan had two clear chances to force a replay. The first came in the 73rd minute when good work from Paul Cairney allowed right-back Paul Paton time to hang a cross to the back post but the substitute could only head into Pernis' hands from four yards.
Buchanan was unmarked again in the box with 10 minutes left but he sliced a volley wide from 12 yards after Donnelly had headed down Archibald's cross.
Thistle's resistance ended with 10 seconds of normal time remaining when United sub Goodwillie latched on to a hopeful long ball, cut in from the left and converted from a tight angle.