U's captain Leon Legge struck what proved to be the decisive goal after 29 minutes, powering a header beyond Christy Pym from Piero Mingoia's free-kick.
Exeter had earlier spurned a glorious chance to go ahead when Joel Grant blazed over after Ollie Watkins' shot was deflected into his path in space.
Luke Berry almost made it 2-0 six minutes before half-time when a kick from goalkeeper Will Norris ran all the way through to him, only for his effort to land on the roof of the net before Norris produced a magnificent point-blank save to keep out Reuben Reid's header.
Needing a result to solidify their own play-off spot, Exeter saw a Grant effort blocked by Legge and Lee Holmes volleyed well wide when a clearance fell to him.
Eleven minutes from the end Norris made another critical save to parry a header from the unmarked David Wheeler to preserve the lead for Cambridge, who are a point from the top seven.
Match report supplied by the Press Association.
Match ends, Cambridge United 1, Exeter City 0.
Second Half ends, Cambridge United 1, Exeter City 0.
Corner,  Exeter City. Conceded by Harrison Dunk.
Jake Carroll (Cambridge United) is shown the yellow card for a bad foul.
Lee Holmes (Exeter City) wins a free kick in the defensive half.
Foul by Jake Carroll (Cambridge United).
Attempt saved. David Wheeler (Exeter City) header from the centre of the box is saved in the top left corner.
Corner,  Exeter City. Conceded by Piero Mingoia.
Luke Croll (Exeter City) is shown the yellow card for a bad foul.
Adam McGurk (Cambridge United) wins a free kick in the defensive half.
Foul by Luke Croll (Exeter City).
Lloyd James (Exeter City) wins a free kick in the defensive half.
Foul by Luke Berry (Cambridge United).
Lee Holmes (Exeter City) wins a free kick on the left wing.
Foul by Jake Carroll (Cambridge United).
Substitution, Cambridge United. Adam McGurk replaces Liam O'Neil.
Leon Legge (Cambridge United) is shown the yellow card.
Joel Grant (Exeter City) wins a free kick in the attacking half.
Foul by James Dunne (Cambridge United).
Attempt saved. David Wheeler (Exeter City) header from the left side of the six yard box is saved in the bottom right corner.
Corner,  Cambridge United. Conceded by Jack Stacey.
Attempt blocked. Uche Ikpeazu (Cambridge United) right footed shot from the right side of the box is blocked.
Attempt missed. Lee Holmes (Exeter City) left footed shot from outside the box is just a bit too high.
Liam O'Neil (Cambridge United) is shown the yellow card for a bad foul.
Foul by Liam O'Neil (Cambridge United).
Ollie Watkins (Exeter City) wins a free kick in the defensive half.
Attempt blocked. Uche Ikpeazu (Cambridge United) right footed shot from the left side of the box is blocked.
Substitution, Cambridge United. Harrison Dunk replaces Medy Elito.
Attempt blocked. Joel Grant (Exeter City) right footed shot from the centre of the box is blocked.
Foul by James Dunne (Cambridge United).
Joel Grant (Exeter City) wins a free kick in the attacking half.
Substitution, Exeter City. Lee Holmes replaces Reuben Reid.
Attempt missed. Liam O'Neil (Cambridge United) header from outside the box is close, but misses to the right.
Substitution, Cambridge United. James Dunne replaces Conor Newton.
Medy Elito (Cambridge United) wins a free kick on the left wing.
Foul by Luke Croll (Exeter City).
Attempt missed. Luke Croll (Exeter City) right footed shot from very close range is close, but misses to the right following a corner.
Corner,  Exeter City. Conceded by Jake Carroll.
Attempt missed. Reuben Reid (Exeter City) header from the centre of the box misses to the left.
Attempt blocked. Uche Ikpeazu (Cambridge United) right footed shot from outside the box is blocked.
Cambridge maintained their League Two play-off hopes with a narrow win over fellow challengers Exeter.