Donald fired a level-par 71 on Sunday to finish on two-over par 286 for 72 holes, sharing 37th place, 16 strokes behind winner Carl Pettersson of Sweden.
Donald needed to share eighth place or better this week to keep his razor-thin margin ahead of McIlroy in the rankings.
He finished six strokes adrift of that mark after matching his highest score in seven Heritage starts.
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