Millwall's winner came after 63 minutes when veteran striker Steve Claridge picked the ball up on the left and fed Ifill, who strode forward to hit a rasping drive low past goalkeeper Paddy Kenny and into the corner of the net.
The goal was just about deserved for Millwall, who had been profligate with their shooting and rarely made Kenny work despite peppering the visitors' goal with shots and headers.
The result stretched their unbeaten run to six games and maintained their position among a host of teams bidding for a play-off spot.
Blades boss Warnock threw on two fresh strikers in Steve Kabba and Wayne Allison as soon as Millwall scored.
Former Crystal Palace frontman Kabba was soon in the thick of the action, volleying over as United went in search of a quick equaliser.
Then young Michael Tonge, back in the United side after a ban, went close when his shot was saved by Millwall goalkeeper Tony Warner.
But the home side held on for victory, avenging a 3-1 defeat at Bramall Lane in August, when their goal was again scored by Ifill.
United had begun the game well, Australian centre-back Shaun Murphy going up for a free-kick and getting in a header that was cleared, before Michael Brown tried one of his trademark 25-yard efforts that Warner kept out.
Millwall got their first sight of goal in the 17th minute, but Murphy blocked a close-range effort from 18-year-old Scot Peter Sweeney, who was making his first full start for the club.
United goalkeeper Kenny, recalled after being rested for last week's FA Cup victory over Ipswich, had been untroubled for the first half-hour and Darren Ward could not test him when his header from Dennis Wise's free-kick sailed over the crossbar.
And when Sweeney's cross was cleared by Murphy, the ball fell to Robert Ryan, but his left-foot volley cleared the crossbar as neither side could achieve the breakthrough.
Millwall were no better at hitting the target in the second half and after David Livermore fired a 25-yard shot wide, Christophe Kinet - on for Sweeney - volleyed over with Kenny a spectator.
Then Paul Robinson got his head to Kinet's far-post corner but knocked the ball wide, before Ifill stepped in with the decisive strike.
Warnock's last throw of the dice was to send on striker Paul Peschisolido, but there was to be no late drama this time.