Gillingham’s play-off hopes suffered a damaging blow as they struggled for spark at Chesterfield.
Early goals from Sam Clucas and Jay O’Shea put Chesterfield in charge at the Proact Stadium.
Clucas finished the game off with Chesterfield’s third with a minute to go.
The hosts remained in command throughout as Gills boss Justin Edinburgh’s unbeaten start on the road ended.
After three games without a win Gills' hopes of making the top six now seem a distant hope.
Edinburgh named an unchanged starting XI following the impressive weekend performance at Bristol City.
The only change in the matchday squad came on the bench, where Jake Hessenthaler returned from a bout of tonsillitis, replacing Mahlon Romeo.
Just one point separated the sides before kickoff. Chesterfield sat three places above the Gills in 11 and both clubs were hoping to make a last-gasp charge for the League 1 play-offs.
Saturday’s defensive performance was arguably Gills’ best of the season but within 12 minutes of Tuesday’s game they were two down.
It took just three minutes for the hosts to take the lead as Clucas ran onto a ball over the top of the defence before finishing well past Stuart Nelson.
Gills’ defence was caught out once again with 12 minutes gone. Clucas’ flick put Gary Roberts in space and he outsprinted the defence before his pinpoint cross was turned home by O’Shea.
The home pace caused Gills defence problems throughout the half and Edinburgh’s side’s best chance of a goal came from set-pieces.
Twice they came within range from the edge of the box but Joe Martin bent one wide and Bradley Dack put another chance the wrong side of the post.
Chesterfield bossed the battle in midfield with the experienced Gary Roberts and Irishman O’Shea getting the better of the Gills.
Gillingham were forced into a reshuffle at the back when Martin limped off injured. Aaron Morris dropped to right-back and Gavin Hoyte moved to the opposite flank.
With ten minutes of the second half Edinburgh introduced Jermaine McGlashan, hoping the extra pace would create more opportunities for the Gills. Departing Oliver Muldoon had got a knock moments earlier following a challenge from Tendayi Darikwa.
McDonald glanced a header wide from a McGlashan cross soon after the winger came on. Doug Loft had a half chance from the edge of the box, after a good bit of possession, but the weak effort on his right foot was scooped up by keeper Lee.
Ollie Banks fired narrowly wide for the hosts as they continued to look comfortable. Clucas also had a shot blocked after he got the better of defender Max Ehmer.
Gillingham’s miserable night ended by conceding a late third when Roberts capped a good night, getting the better of the defence and unselfishly finding Clucas, who passed the ball through Nelson’s legs.
Gillingham: Nelson, Hoyte, Martin (Hessenthaler 31mins), Egan, Ehmer, A Morris, Muldoon (McGlashan 55mins), Dack, Loft, Marquis (Norris 72mins), McDonald. Subs not used: G Morris, Legge, Galbraith, Dickenson.
Chesterfield: Lee, Evatt, Talbot, Hird, Darikwa, Ryan, Banks (Dieseruvwe 90mins), O'Shea, Ariyibi (Humphreys 83mins), Roberts (Onovwigun 90mins), Clucas. Subs not used: Chapman, Carter, Jones, Raglan.
Attendance: 4,804 (179 away)