Midfielder Harley settled a dull encounter on 68 minutes as he received Rob Edwards' fine pass to drill a left-footed shot across Bournemouth goalkeeper Ryan Pryce.
The goal was exactly what Exeter deserved as they pressed and harried their more established opponents, who looked nervous and lacking thrust by only fielding one forward from the start.
Bournemouth though did begin brightly with their five-man midfield stringing some consistent passing movements together only to be caught lacking in the final third as a well-oiled Exeter defence dealt with everything they could throw at them.
At the other end, Exeter striker Adam Stansfield twice broke clear of a Cherries backline, but on both occasions he skewed his shot wide.
As they grew in confidence, Harley then tested teenager Pryce with a stinging 20-yard drive which the Bournemouth goalkeeper initially spilled but recovered in time to deny Marcus Stewart.
Edwards' drive from range was perhaps a lot closer than Pryce first anticipated as he watched it fly past his left-hand post.
When the goal arrived there was more than an air of inevitability as Bournemouth seemed to lose any attacking intent they first had and retreated further into their defensive shell.
Just five minutes before the opener, defender Matthew Taylor wandered unnoticed into the penalty area only to toe a Harley corner just wide.
The home side failed to learn their lesson and were punished thereafter as Harley found room, took one touch to steady himself from Edwards' incisive ball and another to launch his grass-cutter past the outstretched arms of Pryce.
The home crowd gave an ironic cheer on 75 minutes when Brett Pitman claimed their first shot on target, but predictably it was saved by Paul Jones and that was as positive as it got for disappointing Bournemouth.