JOHANNESBURG, 7 Dec 2014 (AFP) - Willett, 27, fired a final round six under par 66 at the Gary Player Country Club to add the Sun City title to his only previous triumph at the BMW International Open in 2012.
He followed up his 65 on Saturday with another dominant display on Sunday which featured six birdies to finish the tournament on 18 under.
Willett edged out playing partners Ross Fisher, who won the Tshwane Open in South Africa earlier this year, and former world number one Donald.
Fisher had seven birdies and three bogeys in a four under par 68 to finish two shots ahead of Donald.
"South Africa is a place that's suited me in the past. I've played good golf down here, love the place, love the people, everything about it is great. It's been a truly fun test of golf," said Willett.
Donald failed to mark his 37th birthday with his first victory since the BMW PGA Championship in 2012 following a frustrating one over par 73.
"There were a lot of positives. It's nice to put myself back in position. I could have played a bit better but I still would have had to shoot five under just to tie with Danny," said Donald.
"He played really focused golf today and hats off, he's a deserved winner."
German Marcel Siem sat three shots behind third placed Donald on nine under after matching his opening round 68 while Thailand’s Kiradech Aphibarnrat was two further back.
Final round scores in the Nedbank Golf Challenge at the Gary Player Country Club, Sun City, South Africa on Sunday (GBR/IRL unless stated; par 72):
270 - Danny Willett 71 68 65 66
274 - Ross Fisher 66 70 70 68
276 - Luke Donald 71 63 69 73
279 - Marcel Siem (GER) 68 72 71 68
281 - Kiradech Aphibarnrat (THA) 72 73 68 68
282 - Miguel Angel Jimenez (ESP) 70 74 69 69
285 - Louis Oosthuizen (RSA) 70 73 69 73, Tim Clark (RSA) 70 71 72 72, Thongchai Jaidee (THA) 71 70 71 73, Jonas Blixt (SWE) 71 73 70 71
286 - Shane Lowry 72 72 71 71
287 - Stephen Gallacher 70 73 72 72, Brendon Todd (USA) 73 68 75 71
289 - Charl Schwartzel (RSA) 70 71 75 73, Tommy Fleetwood 74 67 74 74
290 - George Coetzee (RSA) 68 74 76 72, Pablo Larrazabal (ESP) 73 71 71 75, Lee Westwood 72 70 78 70
291 - Brooks Koepka (USA) 70 74 75 72, Dawie Van Der Walt (RSA) 73 72 69 77
292 - Jaco Ahlers (RSA) 74 75 70 73, Martin Kaymer (GER) 74 75 71 72, Alexander Levy (FRA) 68 70 76 78
293 - Joost Luiten (NED) 72 74 73 74, Thomas Bjorn (DEN) 80 72 68 73
295 - Kevin Na (USA) 77 73 72 73, Marc Warren 75 73 71 76
298 - Mikko Ilonen (FIN) 76 78 72 72
301 - Danie Van Tonder (RSA) 77 78 76 70