The victory was the 16th on the PGA Tour for Els, but his first since 2004. Els finished at 6-under-par 274.
Donald, who had shared the third-round lead, shot a 71 in the final round and just missed a birdie chip that would have forced a playoff with Els.
"I was actually pleased when I saw some breeze this morning when I got to the range," Els said. "I really played well today."
Els won $900,000.
Nathan Green closed with a 67 and placed third, another shot back.