The Scotland international’s strike means Darren Ferguson’s hosts are still not mathematically safe from relegation, although Oldham’s 3-0 defeat to Southend makes that a highly-improbable prospect.
Tony Mowbray’s men will join Wigan Athletic in the second tier following an unbeaten run of 14 matches stretching back to the first Saturday in February, with the Lancashire rivals now competing to see who can finish top.
Blackburn, roared on by just under 4,000 travelling supporters, set the early pace at the Keepmoat and could have been 2-0 to the good after 15 minutes.
First Danny Graham burst through the right channel and rounded Marko Marosi, but, with the angle narrowing, his shot rolled across the goal-line. Next Adam Armstrong’s low ten-yard drive was somehow kept out by a combination of Marosi and Andy Butler.
Mulgrew then called Marosi into action with a well-struck free-kick, while a rare home raid saw David Raya save smartly at his near post from Alfie Beestin after clever play down the right by James Coppinger.
A controversial start to the second half saw play pulled back and Mulgrew cautioned for a foul after John Marquis had burst clear on goal.
Defending brilliantly at the other end, Mathieu Baudry made a superb stretching interception with Bradley Dack poised to tap in. Mulgrew finally made the breakthrough on 80 minutes, just after news filtered through that promotion rivals Shrewsbury had taken the lead against Peterborough. The defender rose highest in the penalty box to beat Marosi with a looping header from Dack’s left-wing corner in front of an ecstatic away end.
Doncaster Rovers: Marosi; Alcock (Kongolo, 83), Baudry, Butler, Mason; Blair, Houghton, Coppinger (McCullough, 73), Whiteman; Marquis, Beestin (Williams, 78). Unused substitutes: Lawlor, Boyle, Anderson, May.
Blackburn Rovers: Raya; Bennett, Lenihan, Mulgrew, Williams; Armstrong (Antonsson, 79), Evans, Smallwood, Conway (Payne, 75); Dack, Graham (Samuel, 75). Unused substitutes: Leutwiler, Whittingham, Bell, Downing.
Referee: Eddie Ilderton ((Tyne & Wear).