Reed started the final round two shots clear of Jason Dufner and extended his advantage early on the final day. But Donaldson and Watson closed the gap as Reed stalled in the mid-part of his round.
Donaldson reduced the deficit to just one with a birdie at the 17th after Reed had missed a short par putt on the 14th. But the Welshman bogeyed the last after finding a bunker with his second shot and Reed was two clear once again.
38-year-old Donaldson joined Bubba Watson on three-under-par in the clubhouse, and the pair could only watch and wait as Reed attempted to close out the victory.
He produced an excellent two putt on the 17th to give himself a two-shot cushion on the final hole and he took advantage of that, laying up with his second shot and two-putting for a bogey five. At just 23-years-old, he is the youngest ever winner of a WGC event, breaking the record of Tiger Woods.
"It means a lot to come out here and play as well as I did, with Tiger close to the lead and Hunter Mahan really close, Jason Dufner all those great guys that are Major winners who have been on Ryder Cup teams. They are just outstanding players, he said.
"I have a lot of confidence in my game. It's one of those things that you build confidence by how hard you work, and I feel like I'm one of the hardest workers out here and it definitely shows."
It was a good week for British players, although it was perhaps not the biggest names that produced the best performances. Jamie Donaldson was the top European in a tie for second and he has bolstered his chances of securing a spot in Paul McGinley's Ryder Cup team this September, he's now second on the European points list.
"It's nice to come here and have a great week on what is a very tough golf course," he said.
Scotland's Stephen Gallacher ended the week in a tie for sixth place after a closing round of 70 and Graeme McDowell was the next best placed Brit in tied ninth.
WGC-Cadillac Championship Trump National Doral, Florida Mar 6-9, purse $9,000,000, par 72
1 Patrick Reed (USA) 68 75 69 72 284 $1,530,000 T2 Bubba Watson (USA) 73 72 72 68 285 $753,000 T2 Jamie Donaldson (Wal) 74 70 71 70 285 $753,000 T4 Richard Sterne (RSA) 74 73 70 71 288 $395,000 T4 Dustin Johnson (USA) 69 74 73 72 288 $395,000 T6 Thongchai Jaidee (Tha) 73 74 74 68 289 $248,333 T6 Stephen Gallacher (Sco) 75 75 70 69 289 $248,333 T6 Bill Haas (USA) 73 76 69 71 289 $248,333 T9 Charl Schwartzel (RSA) 70 76 76 68 290 $151,250 T9 Graeme McDowell (NIR) 73 71 73 73 290 $151,250 T9 Hunter Mahan (USA) 69 74 71 76 290 $151,250 T9 Jason Dufner (USA) 69 77 68 76 290 $151,250
Note: Player score in bold signifies Titleist ball usage Where next? PGA Tour - Chesson Hadley wins Puerto Rico Open
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Fergus is Golf Monthly's resident expert on the history of the game and has written extensively on that subject. He is a golf obsessive and 1-handicapper. Growing up in the North East of Scotland, golf runs through his veins and his passion for the sport was bolstered during his time at St Andrews university studying history. He went on to earn a post graduate diploma from the London School of Journalism. Fergus has worked for Golf Monthly since 2004 and has written two books on the game; "Great Golf Debates" together with Jezz Ellwood of Golf Monthly and the history section of "The Ultimate Golf Book" together with Neil Tappin , also of Golf Monthly.
Fergus once shanked a ball from just over Granny Clark's Wynd on the 18th of the Old Course that struck the St Andrews Golf Club and rebounded into the Valley of Sin, from where he saved par. Who says there's no golfing god?
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