D'Ath's fine exchange of passes with Darryl Duffy and subsequent clinical finish put Cheltenham ahead in the 55th minute after a goalless but entertaining first half.
And it was left for a brace from full-back Keith Lowe to wrap up victory, with third-placed Town having tasted just one defeat in their last nine while the Grecians suffered a fourth defeat in five winless matches.
Alan Bennett was denied by Artur Krysiak in the third minute. At the other end Scott Brown denied Jimmy Keohane, Steve Tully and Liam Sercombe while former Exeter man Billy Jones cleared a Jamie Cureton effort off the line. But Cheltenham ran away with it after the break.
After D'Ath's opener Brown did well to block a swerving drive from Cureton before Lowe made it two-nil. The right-back reached Steve Elliott's flick-on from Jones' corner to force the ball over the line from close range.
And Lowe made sure of the points with a second tap-in after Elliott nodded down another Jones corner in the 87nd minute while substitute Jeff Goulding saw a late effort hit a post.