The visitors looked to have secured their first point of the campaign when Jake Jervis cancelled out Nadir Ciftci’s first half goal for United.
With just four minutes of the match remaining, substitute Erskine caught Antonio Reguero off his line with a superb left-foot shot after the County keeper punched the ball into his path.
United opened the scoring on 19 minutes after Ciftci had been fouled by Yoann Arquin, and from the resulting free-kick the unmarked striker sent his header into the far corner.
Six minutes after the break County levelled as Joe Cardle and Graham Carey opened up the United defence and Jervis arrived at the back post to stab the ball into the net.
The home side made the first chance after six minutes as Scott Boyd blocked Ciftci’s attempt at goal and John Rankin followed up with the rebound which was turned on to the bar by Reguero.
Two minutes after the opening goal Ciftci unleashed a long-range effort which Reguero did well to turn round the post.
United almost restored their earlier advantage when Mario Bilate came close for the hosts as his left-foot shot from inside the box went just past Reguero’s far post.
However, County missed a sitter on the hour. Cardle passed to Uros Celcer, whose cross was met by Arquin but he headed wide from a few yards with the goal at his mercy.
Jervis had a golden opportunity to give County the lead, but his volley from close-range flew high over the target.