The visitors took the lead in the 17th minute when Billy Fogarty curled a cross behind The Ducks' defence for a well placed Gary Julians to head past United keeper Keith Baker.
Then in the 32nd minute the hosts were level when Davy Jones' cross from the right found Chris Nash and as the ball dropped clear, Billy Holmes rifled a superb finish past Darts keeper Keen.
Within a minute it was 2-1 United when Mick Martin chipped forward for Gary Harthill to take it on and Holmes was there to finish it all off.
Then in the 50th minute Dartford equalised when the most amazing goalmouth scamble saw 3 remarkable stops by Baker but there was no clearance to help him out and Willie O'Sullivan almost broke the back of the net with his finish.
In the 85th minute Fogarty's cross for once found Baker wanting, the ball snaked across the goal and Julians fired home the winner of a game that was an advert for Southern League football.