The striker's header in the second minute of stoppage time condemned Heracles to a third consecutive Eredivisie defeat on Friday and Jan de Jonge's men remain bottom of the Dutch top flight.
Oussama Tannane's second-half dismissal presented the home side with an uphill task and Hemmen left it late to wrap up the points for Cambuur with a near-post header from a corner.
The visitors suffered a fright of their own when Wout Droste was left prone on the turf after a poor challenge from Fahd Aktaou.
He was forced from the field after 53 minutes and taken to hospital, while Aktaou, who had already been booked, escaped further sanction.
Cambuur remain unbeaten, having drawn their opening two league games.