The son of Galileo took the Listed honours over a mile and five furlongs, just six days after opening his account at Leopardstown, with his racecourse debut coming only last month.
He is likely to head to Royal Ascot next for the Queen's Vase, with connections feeling he could develop into a contender for the Leger at Doncaster in September.
British raider Yabass, trained by Archie Watson, set a steady pace with Donnacha O'Brien never too far away on the 4/7 favourite, trained by his father, Aidan.
He pushed Southern France two and a half furlongs out to make his horse's stamina count. Drapers Guild challenged strongly in the final furlong, but Southern France held the aces to prevail by half a length.
Speaking from Newbury, where he saddled Rhododendron to win the Group One Lockinge Stakes, O'Brien said: "He is a lovely big horse and he will have no bother going up to a mile and six at Ascot for the Queen's Vase.
"He is babyish and we ran him back a little bit quick, but he needed to learn a little bit if he was going to go to Ascot. We are delighted with him."
O'Brien believes Southern France is the right type for the St Leger.
He added: "He is a lovely, massive, big uncomplicated horse at home. He is very big and is still a baby. We think he could be a St Leger horse."
The winning rider said: "He stays well. We weren't going very quick so I just wanted to make it into a bit of a test. Once he got to the front he had a bit of a look.
"I think he will be a better horse again next year when he strengthens up a bit more, as he's a little weak behind the saddle.
"The Queen's Vase could be a natural enough next step for him."
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