After just six minutes Grindlay came for a cross ball and took the catch cleanly. He opted for a quick kick upfield but blasted the ball off the back of team mate Mark Dempsie, leaving Darren Gribben, making his first full start for Forfar, to gleefully tuck the ball home.
Dumbarton settled, however, and enjoyed the lion's share of posession with Paul Ronald forcing a fine save out of Forfar keeper Francois Dubourdeau.
Forfar shoudl have extended their lead five minutes before the interval when Martin Cameron sent a perfect pass through to Ricky Waddell, but he missed the target from inside the box.
Forfar made sure of all three points in the 64th minute when David Dunn pounced on a half clearance by the Dumbarton defence and hit an angled low drive across the keeper and into the net.
Moments later Martin Cameron rattled the cross bar with a shot form just outside the box.
Ryan Borris almost netted a late consolation, but Dubourdeau pushed his shot over the bar.