The young South African had eight birdies and one bogey to take his place atop the leaderboard. After earning a spot in the field through the pre-qualifying tournament, Clark dominated the final two rounds to earn his first win on the European Tour.
'I just wanted to come out and show I deserved a spot in the tournament,' said Clark, who became the first player to win after pre-qualifying for a tournament since Paul Lawrie captured the British Open in 1999.
Clark began the day with a two-stroke advantage over Retief Goosen and quickly added to the margin with birdies on the second and third. He birdied the eighth and ninth to make the turn at 16-under.
The 26-year-old birdied the first hole on the back nine before making his only bogey of the day on the 11th. Clark responded with a birdie on the following hole and secured the win with back-to-back birdies from the 14th.
Clark played his way onto the PGA Tour last year after finishing third on the Buy.Com money list in 2000. However, a wrist injury that required surgery forced him to miss a large part of the season.
This year he will carry momentum onto the American circuit and will look to earn his first win on that tour at the Pebble Beach National Pro-Am.
'This helps my confidence for the U.S. PGA tour. I am not sure about the wrist yet, but it seems fine at the moment.'
Webster came from three shots back at the beginning of the round to pose a threat to Clark and fired the low score of the day with an 8-under 64, a mark matched only by David Park of Wales.
In an attempt to become the first Englishman to win the South African Open since 1970, Webster eagled the par-5 third but dropped a shot with a bogey on the fourth. He birdied three of the next four holes but remained three shots off the lead.
'Every time I looked at the board, Tim had birdied again, so all credit to him,' said Webster.
He came within one stroke of Clark after back-to-back birdies from the 13th, but eventually fell short in his attempt to earn his first victory on the European tour and finished at 17-under 271.
'I do feel the first win is just around the corner,' he said. 'I always play well in South Africa and Houghton next week is one of my favorite courses. I am looking forward to the Dunhill Championship.'
James Kingston, who led after the second round, had six birdies and two bogeys for a round of 68. The South African finished alongside England's Jonathan Lomas at 13-under 275.
U.S. Open champion Retief Goosen, who trailed Clark by two strokes to begin the final round, managed a two-under 70 to finish at 12-under 276. He was joined by Park, England's Simon Dyson (67) and Alastair Forsyth (69) of Scotland.
Full-Field Scores from the Bell's South African Open