John Melligan's first goal of the season earned Cheltenham the points against 10-man Millwall.
The home side went ahead with Grant McCann's disputed penalty after Paul Robinson's challenge on Steve Guinan.
Millwall drew level with a brilliant 25-yard strike from Derek McInnes, but keeper Lenny Pidgeley then saw red for deliberate handball outside the area.
The visitors went ahead through Marvin Williams' low shot but Kayode Odejayi and Melligan sealed Cheltenham's win.
Cheltenham: Higgs, Gill (Vincent 61), Caines, Victory, Armstrong, Melligan, Finnigan, McCann, Wilson, Odejayi, Guinan (Gillespie 61), Gillespie (Bell 76).
Subs Not Used: Brown, Wylde.
Goals: McCann 14 pen, Odejayi 67, Melligan 82.
Millwall: Pidgeley, Ross, Robinson, Whitbread, Shaw, Hackett (Ardley 47), Elliott, McInnes, Morais (Day 38), Williams (Hubertz 72), Braniff.
Subs Not Used: Craig, Mawene.
Sent Off: Pidgeley (36).
Booked: Williams, McInnes, Hackett.
Goals: McInnes 24, Williams 45.
Ref: T Kettle (Berkshire).