Goals from David Goodwillie and Andy Webster either side of the break ensured the Tannadice side would be back at Hampden on May 15 to do battle with Celtic's conquerors in a bid to land the silverware.
And today's result means County are now likely to have European football to look forward to next season if United continue their impressive form in the Scottish Premier League to secure a Europe League spot through their league position.
Events in the other semi-final at the same venue 24 hours earlier meant the outcome of this game was impossible to predict beforehand and the opening spell was scrappy with both teams taking time to settle.
Raith carved out the first real chance when Stephen Simmons stabbed into the path of Allan Walker who found himself with only the goalkeeper to beat from close range but Dusan Pernis was up to the challenge and did well to block.
United suffered a setback when they were forced to make a change with just 16 minutes on the clock when Paul Dixon limped out of the action and was replaced by Keith Watson.
The Kirkcaldy side had appeared hungrier and looked dangerous early on but it was their SPL opponents who opened the scoring after 28 minutes.
Danny Swanson played a lovely ball down the right flank for Goodwillie and he did well to round goalkeeper David McGurn before slotting into the back of the net from the tightest of angles.
United could have added to their lead before the interval. Swanson was the provider again, this time with a flick-on for Jon Daly who somehow managed to blast over from eight yards when it looked easier to leave the net bulging.
Peter Houston's men emerged for the second half keen to snuff out any hope of a Raith comeback and could have had a penalty when Dougie Hill appeared to handle in the box but their pleas were ignored by referee Craig Thomson.
Then, only a timely intervention from Thierry Gathuessi prevented Daly from connecting with a long ball from Watson a few yards out as United continued to press for a second goal.
United's persistence paid off and they doubled their advantage after 58 minutes when Craig Conway's corner picked out Webster and he rose above everyone else to bullet a header past the helpless McGurn, with there now appearing to be no way back for Raith.
To their credit, the plucky Fifers tried to haul themselves back into the tie and were not too far away when Jamie Mole threw himself in front of Walker's free-kick only to direct his header just wide of Pernis' right-hand post.
The United goalkeeper was then called into action to smother a long-range low effort from Simmons at the near upright.
But there was to be no all-First Division final as United held on to secure safe passage to the showpiece event.