The striker’s first goal since rejoining the club answered a Tope Obadeyi opener (20) moments after Kilmarnock had lost Darryl Westlake to a 45th-minute red card.
A winning goal would not come even against Killie’s 10 men but a point, on the back of last week’s win over Inverness, was enough to take Ian Baraclough’s side above St Mirren.
On another day Motherwell might have won comfortably, but they came up against an inspired opponent in the visitors’ goalkeeper Craig Samson, who was more responsible than most for a sixth straight league game without defeat.
McDonald alone was denied three times in the first half, Samson’s stop to keep out his close-range header the pick of several top-class stops.
In among all the ‘Well chances it was Killie who went ahead, top scorer Obadeyi shifting the ball on to his left foot and beating George Long with a powerful shot.
But McDonald finally got his goal, curling in a low free-kick awarded when Westlake fouled Marvin Johnson to collect his second booking.
Johnson forced Samson into another decent save after the break, but the keeper had a quieter time of it as his side dug in and even came close at the other end, Nathan Eccleston firing just wide.