Els, playing in his home country, had nine birdies and finished at 21-under 267 total on the 7,743-yard Gary Player Country Club to claim the $2 million first prize, the richest in golf.
'That was possibly one of the great rounds of my career,' said Els, who won Nedbank for the third time in four years. 'I wasn't going after 63 when I started. I just needed to get a decent start but when Colin and I got onto the homeward nine, we really just lit it up.
'Today was quite nice. I started out a little worried about Monty, because he was playing quite well.'
Montgomerie, who moved into contention with a third-round 65, had three birdies and an eagle on the back nine, but could not stop Els.
'He said he tried really hard to chase me down, but there was nothing he could do about it,' Els said.
Chris DiMarco shot a 2-under 70 and finished third at 10 under. Jim Furyk closed with a 69 and tied Retief Goosen for fourth at 7 under.
Defending champion Sergio Garcia had five bogeys in a round of 2-under 70 and was sixth at 5-under 283.
Michael Campbell hit only one fairway -- the last -- in a disastrous final round and finished last at 289 after a 78.
Full-field scores from the Nedbank Golf Challenge