But it was the visitors took the lead from the penalty spot on five minutes after Stuart Annetts had handled the ball inside the box.
Ryan Donnelly converted the spot kick.
But Lanark were also awarded a penalty on 15 minutes after Ronan Kearney was fouled inside the area.
Shaun Gallagher beat the keeper at his left post to level the scores.
Annetts then had a chance from a Gallagher lay-off on 32 minutes but sent his shot well wide of the target.
Gartcairn left back Murray Spooner sent over an inviting cross on 35 minutes but the ball was headed wide of the post.
Kearney broke away on the left on the hour mark but there were no takers for his low ball across the face of the goal.
United took the lead on 75 minutes when Jack Stubbs drove the ball into the net following a right wing corner.
Five minutes later Donnelly scored the equaliser which should never have stood as he was clearly offside but the goal was allowed despite United protests.
This gave the visitors a boost and substitute Kevin Ross gave them the lead when he headed in a left wing free kick on 85 minutes.
United attacked strongly in the closing stages and were rewarded with a last minute goal scored by Peter O’Donnell in a crowded area to give his side a well earned share of the points.
United’s ‘never say die’ spirit was praised by gaffer Paul Burns.
He told the Gazette: “That is two weeks in a row we have got a last minute equaliser.
“So it does say a lot about the players’ character and willingness to go to the end.
“It’s good to see the players fight, although in the long term I hope we’re not relying on last gasp goals a lot.”
Although they’re down in eighth place in McBookie.com Central Division 2, United are currently just four points behind third placed Vale of Clyde and the promotion positions.
And, after a recent run of fixtures against top teams in the division, United now have Vale of Leven, Newmains, Ashfield and Dunipace in their next four fixtures to seemingly give them a great chance of picking up points and climbing the table.
“These games could make or break our season,” Burns said.
“We will be going out to win every game.”