'I'm very excited and very, very happy,' said Zhang, who became the fourth consecutive first-time winner on the 2003 European Tour schedule.
Zhang fashioned a final-round 2-under 70 on Sunday to finish at 10-under-par 278, one shot better than Els as the tournament came down to a thrilling conclusion.
Zhang and Els were tied at 10-under par after Els bogeyed the 14th hole but Zhang made the first mistake after that. At the 202-yard par-3 17th, Zhang pulled his tee shot left of the green and carded a bogey-4 to fall one behind Els as they made their way to the 72nd hole.
Els pushed his tee shot right at 18, bouncing off a cart path and a tree before landing in the rough. His sand wedge came up short of the green but he pitched to five feet.
Zhang's drive flirted with a water hazard but he left himself with a 9-iron approach. His second shot nestled four feet from the cup, giving him the opportunity to make birdie and possibly match Els if the No. 2 player in the world converted his par save.
Els' putt for par lipped out of the hole and Zhang drained his putt to claim victory.
'Ernie Els is a great golfer and it was a great pleasure to play with him,' said Zhang. 'We had a great day and I just kept trying my best. To be honest it wasn't until the 18th green that I thought I had a chance to win.'
Els had a chance to earn his third international win in as many weeks. He dominated the field in a record-setting performance at the PGA Tour's Mercedes Championships two weeks ago and defeated Aaron Baddeley in a playoff last week at the Sony Open in Hawaii.
The reigning British Open champion struggled in the intense heat at Laguna National Golf and Country Club and the worldwide schedule finally caught up to Els.
'I have to say that the heat eventually got to me a little bit,' said Els, who is off to Australia next week to defend his title at the Heineken Classic. 'I felt a little bit tired towards the end and it was tough.
'But I take my hat off to Zhang. He stuck in well and didn't make any mistakes. But the way I putted, I let them all in a bit.'
Els squandered a two-shot lead to start the day thanks to a final-round, 1-over-par 73.
Prayad Marksaeng finished alone in third place at 8-under par, followed by Simon Khan and Maarten Lafeber, who shared fourth place at minus-7.
Rick Gibson, Andrew Marshall and Per Nyman tied for sixth at 5-under par. Fran Quinn took ninth at 4-under-par 284.