The Stags scored the only goal of the game on the stroke of half-time before withstanding a second-half barrage from the Grecians, who did everything but score in a one-sided second half.
But that will be of little comfort to Exeter manager Paul Tisdale, whose side have now lost six league games at home this season already.
Mansfield edged the first half, but it was not until the stroke of half-time that they went in front. The Exeter defence failed to deal with a long ball forward and Lee Stevenson danced through before planting a low shot across goal and into the corner.
Exeter laid siege to the Mansfield goal after the break and created several chances to equalise. Tom Nichols fired straight at visiting goalkeeper Alan Marriott, who also saved well from Liam Sercombe and substitute Jimmy Keohane.
The Stags had chances on the counter-attack, but Anthony Howell was twice foiled by Exeter keeper Artur Krysiak and with virtually the last kick of the game, home striker Alan Gow was denied by the inside of the post.