Six minutes later the lead was doubled when Jamie Paterson's shot was blocked by Grant Brebner but fell kindly for McDonald, whose shot somehow managed to trickle under the keeper.
Well were dominant for most of the half, and chances were missed by Jim Hamilton and McCormack before they were stung just before the break. United scored with their first real effort on goal when Lee Miller headed a Stevie Crawford cross past Colin Meldrum from 12 yards, with the Well defence posted missing.
The start to the second half had everything. Motherwell extended their lead after 48 minutes when substitute Marc Fitzpatrick hit home a Hamilton knockdown from six yards with his first touch.
The visitors pulled one back four minutes later when David Fernandez latched on to a loose ball to shoot home from the edge of the box.
The two-goal lead was restored after 56 minutes, as Hamilton headed a Steven Hammell free past Stillie.
It was all action and United pulled to within a goal again after 67 minutes when Miller bulleted a header past Meldrum from a fine right- wing cross from Mark Wilson.
Unbelievably the visitors were level after 70 minutes when Collin Samuel hammered a low cross into the six-yard box and Brebner gleefully hammered the ball home from close range.
Things were to get better for the visitors as they took the lead when Brebner was on hand to force a ball from Miller into the net from close range.