Dumbarton looked set for a crucial win when Sam Stanton netted a double following the dismissal of Ross Docherty.
But United, a man short for more than an hour, hit back through a Michael Rose free-kick and Craig Moore’s last-ditch penalty.
Ayr boss Ian McCall admitted: “I didn’t see it coming. The build up to the second goal especially was absolutely brilliant. I don’t know where it came from.
“It says a lot that they never gave in.
“After 120 minutes the other night on an artificial surface then to get a player sent off after 20 minutes it was very, very hard.
“It could be an important point but we’ve got to turn draws into wins and to do that we’ve got to keep 11 men on the pitch.”
In the 27th minute the proverbial six-pointer got a lot tougher for Ayr as Docherty saw red for a rash challenge on Andy Stirling.
The Sons took advantage on 62 minutes when Stanton found the top corner from the edge of the box before the same player doubled the lead soon after.
Ayr set up a remarkable finale when Rose expertly scored a free-kick and they claimed a point when Moore converted from the spot following Darren Barr’s foul on Jamie Adams.
Dejected Dumbarton boss Stevie Aitken said: “That was self-destruct. We got complacent and so we brought it on ourselves.”