The Australian ended the dream run of Irish Champion Madeline Perry towin the title for a fourth time.
Perry, the first Northern Irish finalist in the history of the women's event, was unable to reproduce the form which saw her trounce hot favourite Nicol David, the World No.1 from Malaysia, in the quarter-finals.
Grinham dominated the first two games and held match-ball at 10-6 in the third. But Perry stepped up a gear and saved four match balls to take the game into a tie-break.
However, the former World No.1 from Australia reclaimed the advantage to run out an 11-6 11-5 12-10 winner after 39 minutes.
"I was really focussed today - I couldn't have played any better," said Grinham.
"It hasn't really sunk in yet. At the beginning of the week I was a few points from getting knocked out - I was lucky to get through," added the Queenslander, ranked four in the world.
"To win the British Open for the fourth time is not easy to do."
"I have been having a lot of trouble with confidence all week - so having a result like this going into the Worlds next week should give me confidence."
Rachael Grinham of Australia defeated Madeline Perry of Northern Ireland 11-6, 11-5, 12-10 to win the women's championship in the internationalSPORTgroup British Open - Manchester 2009 at the National Squash Centre in Manchester.