The only goal of an entertaining game came from Reuben Reid in the fifth minute after Jordan Moore-Taylor's long ball was needlessly handled inside the penalty box by Jake Carroll.
Although Reid saw his penalty saved by David Forde, the ball came straight back to him to tap in the rebound.
David Wheeler saw a header come back off the inside of the post, while Cambridge almost levelled when Exeter goalkeeper Christy Pym completely missed a kick, but he managed to get back to scramble Luke Berry's shot off the line.
Liam McAlinden then missed a golden chance for Exeter, heading wide from six yards, before Lloyd James skied another glorious chance for Exeter high from 10 yards with just Forde to beat.
McAlinden skied another great chance high shortly after half-time before being denied by Forde, while a rare Cambridge attack saw Berry volley over after good work by Uche Ikpeazu.
Match report supplied by the Press Association.
Match ends, Exeter City 1, Cambridge United 0.
Second Half ends, Exeter City 1, Cambridge United 0.
Jake Taylor (Exeter City) wins a free kick in the defensive half.
Foul by Uche Ikpeazu (Cambridge United).
Attempt blocked. Ryan Harley (Exeter City) right footed shot from the centre of the box is blocked.
Attempt missed. Jordan Tillson (Exeter City) right footed shot from the right side of the box is high and wide to the left.
Corner,  Cambridge United. Conceded by Pierce Sweeney.
Foul by Ryan Brunt (Exeter City).
(Cambridge United) wins a free kick in the attacking half.
David Wheeler (Exeter City) wins a free kick on the left wing.
Foul by Liam O'Neil (Cambridge United).
Lloyd James (Exeter City) wins a free kick on the right wing.
Foul by Medy Elito (Cambridge United).
Corner,  Cambridge United. Conceded by Pierce Sweeney.
Foul by David Wheeler (Exeter City).
Jake Carroll (Cambridge United) wins a free kick in the defensive half.
Substitution, Exeter City. Ryan Brunt replaces Reuben Reid.
Foul by Pierce Sweeney (Exeter City).
Harrison Dunk (Cambridge United) wins a free kick on the left wing.
Attempt missed. Jabo Ibehre (Cambridge United) right footed shot from the centre of the box misses to the left.
Jake Taylor (Exeter City) wins a free kick in the defensive half.
Foul by Jabo Ibehre (Cambridge United).
Pierce Sweeney (Exeter City) wins a free kick in the defensive half.
Foul by Harrison Dunk (Cambridge United).
Substitution, Cambridge United. Harrison Dunk replaces Gary Deegan.
Attempt saved. Jabo Ibehre (Cambridge United) header from the centre of the box is saved in the centre of the goal.
Pierce Sweeney (Exeter City) wins a free kick in the attacking half.
Foul by Jabo Ibehre (Cambridge United).
Attempt missed. Luke Berry (Cambridge United) right footed shot from the centre of the box is too high.
Attempt saved. Uche Ikpeazu (Cambridge United) header from the right side of the box is saved in the top left corner.
Substitution, Exeter City. Ryan Harley replaces Liam McAlinden.
Corner,  Exeter City. Conceded by David Forde.
David Wheeler (Exeter City) wins a free kick on the right wing.
Foul by Jake Carroll (Cambridge United).
Attempt saved. Uche Ikpeazu (Cambridge United) left footed shot from the left side of the box is saved in the bottom right corner.
Jordan Moore-Taylor (Exeter City) wins a free kick on the right wing.
Foul by Uche Ikpeazu (Cambridge United).
Attempt saved. Luke Berry (Cambridge United) right footed shot from outside the box is saved in the centre of the goal.
Attempt saved. Liam McAlinden (Exeter City) right footed shot from the left side of the box is saved in the bottom right corner.
Attempt missed. Liam McAlinden (Exeter City) right footed shot from the centre of the box is high and wide to the right.
Last season's League Two play-off finalists Exeter opened the new season with a comfortable win against Cambridge at St James' Park.