Liam Hatch's 10th-minute strike secured a vital three points for Barnet as the Bees held on against nine-man Mansfield to boost their survival hopes.
The forward capped a man-of-the-match display with his first goal this term when he turned in a Nick Bailey corner.
Town were then reduced to nine men when Giles Coke, for his violent reaction to Barry Fuller's challenge, and Danny Reet, for a lunge on Bailey, saw red.
But Town nearly got a draw when Richard Barker struck the post in injury time.
Barnet: Flitney, Hendon, Warhurst, Charles, King, Bailey, Hessenthaler (Batt 90), Sinclair (Vernazza 82), Fuller, Hatch, Kandol.
Subs Not Used: Reed, Gross, Roache.
Booked: King, Hatch, Fuller, Hessenthaler.
Goals: Hatch 11.
Mansfield: Pressman, Buxton, Baptiste, Hjelde, Wilson, Dawson (Uhlenbeek 73), D'Laryea, Coke, Rundle (Lloyd 57), Barker, Russell (Reet 57).
Subs Not Used: Day, White.
Sent Off: Coke (55), Reet (71).
Booked: Coke.
Att: 2,784
Ref: K Wright (Cambridgeshire).