Former Killie striker Jim McIntyre popped up in injury time to steal a victory for Dundee United.
Killie had recovered from the loss of a cheap early goal to equalise midway through the first half, but their efforts to turn their domination into goals cost them dear.
United were gifted the lead in the seventh minute just after the home side should have scored.
Kris Boyd lobbed Alan Mahood's cross into the path of Paul Di Giacomo who should have done better than volley over from 12 yards.
United cleared upfield and there appeared to be no danger as Chris Innes took control 35 yards out. He mis-controlled and Steve Thompson had the simple task of putting the ball past Gordon Marshall into the far corner of the net.
Killie took control of the midfield with Mahood breaking down everything United had to offer and his side almost levelled when Steve Murray sent a flashing volley inches wide.
The equaliser came five minutes later when Steve Fulton drifted in from the right to send a fierce drive low into the net off the near post.
Continuing rain made conditions underfoot difficult with players slipping and sliding all over the pitch.
Boyd almost put Kilmarnock ahead in the 65th minute when his free-kick cannoned off the post, but it was United who had the last word when Shaun Dillion, who had been very impressive in his first start of the season, miscued a header on his own six-yard line in injury time.
The ball fell perfectly for McIntyre who fired home the winner from a tight angle to give United their first win over their hosts since 1997