Skipper Phil Mason lofted a free kick to the far post where Royals' Martin Williams stuck out a foot. The ball rebounded off his right leg past goalkeeper Scott Howie for an own goal.
In the 7th minute United stunned the four figure crowd by going 2-0 up. Allan Pluckrose launched a free kick from the halfway line into the box and Cliff Hercules headed superbly past Howie - his 299th career goal for the club.
Apart from Hercules going close soon afterwards, most of the action for the remainder of the half came at the other end, although Gary Phillips had little to do thanks to his hardworking defence and some wayward shooting by the Royals.
Eleven minutes into the second half Darren Grieves made the most of a mistake by Primus and sprinted clear before firing low past Howie. The ball hit the post and came back to Crawshaw who shot wide of an open goal.
On 64 minutes a Gurney cross was headed onto his own crossbar by Garry Wotton, and three minutes later Phillips did well to save a downward header by McIntyre. Mason sent a dipping shot just over the bar late on, but he didn't have to wait long before he could hold the cup aloft along with his Cup winning team-mates.