Richard Barker bagged the only goal as Mansfield prevented Swansea from moving to the top of League Two.
Barker met Adam Rundle's low cross and powered in a shot from the edge of the box in first-half injury time.
A fiery game saw Swansea's Garry Monk sent off for two fouls in a minute and Stags winger Simon Brown dismissed for head-butting Andy Robinson.
Izzy Iriekpen had a goal disallowed for Swansea while Derek Asamoah's overhead kick went just over for Mansfield.
Mansfield: Pilkington, Neil, Baptiste, Buxton, Rundle, Murray,
Curtis, Brown, Jelleyman, Barker (Smeltz 90),
Asamoah (Larkin 84).
Subs Not Used: White, McLachlan, McIntosh.
Sent Off: Brown (68).
Goals: Barker 45.
Swansea: Murphy, Tate, Monk, Iriekpen, Austin, Britton,
Robinson, Martinez, Anderson (Maylett 60), Connor (Thorpe 60),
Subs Not Used: Jones, Corbisiero, Pritchard.
Sent Off: Monk (63).
Booked: Tate, Monk, Trundle.
Ref: P Taylor (Hertfordshire).