The early season pace-setters had won their previous four league fixtures without conceding but Aiden O'Brien scored the solitary goal for deserved winners Millwall who climbed to 15th.
Leeds are one of three sides locked on 17 points alongside Wolverhampton Wanderers, who beat Nottingham Forest 2-1, and Cardiff City, who drew 1-1 with Sheffield Wednesday.
Preston North End and Ipswich sit two points adrift after 3-1 and 2-0 wins over Birmingham City and Bolton Wanderers while Sheffield United also have 15 points despite a 1-0 loss at Norwich City.
Middlesbrough climbed to seventh with an entertaining 3-2 victory against Queens Park Rangers, Bristol City thumped Derby County 4-1 and Aston Villa cruised to a 3-0 win at Barnsley.
Sunderland and Hull City, both relegated from the Premier League last season, drew 1-1, Burton Albion beat Fulham 2-1 and Reading drew 1-1 at Brentford.
(Reporting by Tom Hayward, editing by Ed Osmond)
