Leicester were beaten 2-1 at Hillsborough as Connor Wickham registered a first-half brace for the team who sacked Dave Jones over the weekend.
Wednesday were behind inside three minutes to a strike from Anthony Knockaert, but Wickham - on loan from Premier League Sunderland - hit back to put his team in front by the 25th minute.
The hosts, under the caretaker stewardship of Stuart Gray, held on in the face of late pressure to claim only their second win of the season.
There were also wins for basement club Barnsley, who dented Brighton and Hove Albion's play-off hopes with a 2-1 away win, and third-bottom Yeovil Town, 1-0 victors over a Blackpool side that finished with eight men.
Barnsley, another side to part company with their manager in recent days, claimed a two-goal lead at Brighton courtesy of goals from Paddy McCourt and Jacob Mellis.
Former England defender Matthew Upson pulled one back with 27 minutes remaining, but the visitors held on.
Yeovil remain a point above Wednesday and Barnsley after Everton loanee John Lundstram grabbed the only goal of the game at Huish Park.
Lundstram struck after 21 minutes and Blackpool's frustration was clear in the closing stages as three players received red cards.
Kirk Broadfoot was dismissed in the 90th minute for a second bookable offence, before Ricardo Fuller and Gary McKenzie also received their marching orders.
QPR avoided an upset, beating Bournemouth 3-0 to join Leicester on 38 points.
Charlie Austin, Junior Hoilett and substitute Matt Phillips were on target at Loftus Road, the former claiming his eighth of the season.
Burnley missed out on the chance to share top spot, their winless run stretching to five league games courtesy of a 0-0 home draw with Watford.
Reading climbed to fourth as Billy Sharp's early goal proved enough to see off Charlton Athletic 1-0, while Nottingham Forest twice came from behind in a 2-2 draw at Millwall.
Elsewhere, Ipswich Town moved up to eighth by beating Blackburn Rovers 3-1 at Portman Road.
Oliver Norwood was on target for successive games as Huddersfield Town won 1-0 at Bolton to claim a third straight win and the meeting between Birmingham City and Doncaster Rovers ended 1-1.
