Englisman Sullivan finished 17 under par at Royal Johannesburg and Kensington Golf Club, two shots ahead of compatriots David Howell and Anthony Wall, Ireland’s Kevin Phelan and South African pair Jaco van Zyl and Wallie Coetsee.
The 27-year-old from Nuneaton, who defeated Charl Schwartzel in a play-off in the South African Open at nearby Glendower in January, also claimed one of the three places up for grabs in the Open Championship at St Andrews, with Howell and Wall claiming the other two by virtue of their higher world ranking.
Sullivan, who lifted the Scottish Stroke-Play title at Blairgowrie in 2011 – he was also on a winning Walker Cup team at Royal Aberdeen that year – began the final round three shots behind overnight leader Coetsee, but moved to the top of a crowded leaderboard with five birdies in a flawless front nine of 32.
A bogey on the 11th dropped the former Walker Cup player a shot behind Howell, who had gone to the turn in 33, only for Howell to hook his tee shot into the water on the 14th.
Wall’s challenge also faltered when he found water on the 15th and it was Sullivan who held his nerve down the closing stretch with birdies on the 15th and 18th to seal victory.
“It’s unbelievable,” Sullivan said at the presentation ceremony. “I never imagined it would happen again so quickly. Coming down the stretch my caddie and I were loving life again and enjoying it.
“It seems to be a theme, enjoying my golf and getting the right results at the moment.”
Sullivan won a trip into space for a hole-in-one during last year’s KLM Open and added: “I think I am already there, to be honest with you. I am on the crest of a wave at the moment and I don’t want it to end. I just want it to keep going.
“I want to say a big thank you to my coach Jamie Gough, who has really turned it around, and my psychologist Lee Crombleholme, who has been exceptional this week, working hard on being patient out there.
“I am just grateful it has all paid off and I get to hold another magnificent trophy in South Africa again.”
He added: “I have never played in an Open Championship before so it will be great to get the first one under my belt. It’s absolutely fantastic to get a spot in The Open at St Andrews.”
Howell has played in 12 previous Opens but will play in golf’s oldest championship at St Andrews for the first time this year on the course where he won the Dunhill Links in 2013.
“It was one of the main reasons for coming here, so to come away with one of the spots is fantastic,” he said. “It means a lot to play again in The Open. It was one of my goals this year.”
An equally delighted Wall said: “It’s the ultimate place to play golf and The Open.”
The Open Qualifying Series continues at the Irish Open and the Mizuno Open, both being played in May.
Andrew McArthur finished as top Scot, closing with a 67 to finish joint-18th on 11-under. Scott Henry (72) ended up three shots behind him in a share of 37th, with Tony Jacklin’s Scottish-born son Sean (71) tying for 66th on two-under.