The first half was a fraught affair with no clear-cut chances and it took an incredible 30-metre lobbed shot from United's David Goodwillie to open the scoring in the 61st minute.
He pounced as County keeper Michael McGovern charged out of his penalty area to try and head a high ball clear. But McGovern only succeeded in sending the ball spinning to Goodwillie on the left who hoisted it straight into the net.
United made it 2-0 with a fabulous individual effort from Craig Conway who shrugged off two challenges as he burst free on a surging run into the box before drilling a low 15-metre shot past the advancing McGovern.
Conway cracked in United's third with four minutes left as he fastened on to a flick from Morgaro Gomis from the right to score from eight metres.
It was County's first national final since being admitted into senior football in 1994 and their Highland town of Dingwall was virtually deserted as most of the 5,000 population headed to Hampden to make up an amazing 20,000 support for the team.
