Three first half goals gave Millwall a lead that turned out to be unassailable as Chesterfield fell to a 3-1 Proact defeat.
The Spireites had opened brightly but were hit on the break when Shane Ferguson gave the vistors the lead, an advantage doubled then tripled before half time by two Steve Morison goals.
Sam Hird pulled a goal back for the Spireites after the break, but they were uable to edge any closer as they lost at home for the first time this season.
Millwall went close to scoring inside the opening minute when Shaun Williams saw his 20-yard strike thump the bar.
The Spireites also threatened to make an early breakthrough when Dan Gardner and Jay O’Shea combined and after Ched Evans mis-kicked, the ball fell to Gboly Ariyibi at the far post, but the winger’s low shot was saved by Jordan Archer.
It was Millwall who scored the opening goal in the 13th minute when, following a good build-up on the right, the ball was played to Shane Ferguson who drilled a low shot from the edge of the area beyond Ryan Fulton.
Chesterfield nearly made the perfect immediate response when Dan Gardner put a 20-yard volley narrowly wide.
As Millwall continued to press, David Worrall headed wide at the far post and Morison saw his shot comfortably saved by Fulton.
The visitors were caught on the break when Jay O’Shea slipped the ball through to Jon Nolan and with Evans making strides into the area, it all looked promising until a Millwall defender got a foot in to make an important clearance.
Morison (38) extended Millwall’s lead, holding off Sam Hird before producing a fine finish from the edge of the area.
Morison struck again three minutes later, beating Ian Evatt to a long ball and expertly lobbing the onrushing Fulton from 30 yards out.
Soon after the re-start, Nolan saw his free-kick from the right deflected over the bar.
The Gardner’s corner went all the way to Evatt at the far side of the area and the defender’s shot was only just beyond the angle of bar and post.
That proved to be Gardner’s final contribution of the afternoon as he was replaced by Reece Mitchell in the 52nd minute.
Chesterfield were having plenty of possession and they needed a goal to spark any hopes of a comeback.
O’Shea got the ball on target but his shot lacked the power to test Archer. Evans also shot straight at the Lions keeper.
Danny Wilson made a second substitution just before the hour-mark with Kristian Dennis taking over from O’Shea.
Hird made an important clearance with Aiden O’Brien lurking inside the area.
The defender then made an important contribution at the other end, scoring with a header in the 65th minute after connecting with Nolan’s free-kick.
Hird was in the thick of the action again, whipping the ball away from the feet of Morison, just when the striker looked set to break free.
With around five minutes remaining, Evans fired in a free-kick which was headed off the line by Tony Craig.
Dennis also put a shot on the turn off-target in the closing stages while Mitchell had an effort charged down.
Chesterfield: Fulton (gk) - Donohue, Evatt, Hird, McGinn - Gardner (Mitchell 52'), Liddle (c), Nolan, Ariyibi - O'Shea (Dennis 59'), Evans
Subs not used: Allinson (gk), Raglan, Dimaio, Simons, Daly
Millwall: Archer (gk), Martin (Onyedinma 38' (Nelson 46')), Craig, Williams, Worrall, Thompson, Romeo, Webster, Ferguson (Butcher 82'), Morison, O'Brien
Subs not used: King, Abdou, Wylde, Philpot
Chesterfield: Hird 66'
Millwall: Ferguson 13', Morison 38' 41'
Attendance: 5,630 (489 away fans)