"We are delighted Branden has chosen IMG to manage all aspects of his career," said the company's global head of golf Guy Kinnings.
"While his achievements on the course have been remarkable, they are certainly matched by his likeable personality and marketability off the course."
Grace grabbed the 11th card at Qualifying School in 2011 and went on to achieve the best record by a graduate in the history of the European Tour.
"Last year was fantastic for me," said the 24-year-old. "I really found my feet on the European Tour.
"I plan to keep my focus there while playing as much as possible on the Sunshine Tour and also to gain experience in the United States to ensure I give myself the best possible chance in the big tournaments."
Grace was beaten 4 and 3 by American Robert Garrigus in the first round of last week's WGC-Accenture Match Play Championship in Arizona.