Poulter started his final round four shots behind joint leaders Louis Oosthuizen and Lee Westwood, but clawed his way to the top after matching the seven-under-par 65 he shot a day earlier to finish on a combined 21 under.
Poulter, who also won the 2010 WGC Accenture Match Play Championship, finished two strokes ahead of South African Ernie Els (67) and U.S. trio Jason Dufner (64), Scott Piercy (65) and Phil Mickelson (68), who had a chance to finish second on his own but missed a five-foot putt on the last.
Westwood and Oosthuizen finished tied for sixth, three shots adrift, after carding matching level par 72s.
Poulter conceded he thought the tournament was Oosthuizen's to lose after the South African burst to the top of the leaderboard at 16 under after the second round.
He could only look on with envy as Lee Westwood fired a 61 and Brandt Snedeker flirted with a 59 in the third round before finally carding a course record 60.
Poulter, however, refused to go away, and made the most of his short game to card back-to-back seven under 65s on Saturday and Sunday to take the title.
"I had seen all the other guys shooting low and felt the way Louis played on the first two days he was going to run away with it," added Poulter.
The 36-year-old was under immense pressure after finding a greenside bunker at the last, but with water just beyond the flag he played a delicate recovery from the sand and rolled home a 10-foot putt to seal the win.
"I knew there was a low round of golf our there in me. My short game has been good this week and at the 18th I knew I could get up and down from anywhere.
"And shooting 14 under over the weekend is a decent score."
Poulter is the only player to have appeared in all eight tournaments since the HSBC Champions was launched at the Sheshan International Club in Shanghai in 2005.
The win at Mission Hill's was his first strokeplay victory since the 2010 Hong Kong Open.
"Its been an amazing six weeks with the Ryder Cup," he said.
"But it's so nice to get back into the winner's zone again. In my career I have only had one season without a victory and I didn't want to go another season without one - that would have been a disappointment."
Earlier it in the day it was announced that HSBC had renewed their contract to sponsor the Champions event for the next three years and it will return to Sheshan in 2013.
Tim Finchem, the US Tour commissioner, also announced that next year's event would count towards the start of the 2014 FedEx Cup in America.
- Sports & Recreation
- Louis Oosthuizen
- Lee Westwood