Electing to bat, India posted 264 for five with Garg, who has been picked by Rajasthan Royals for Rs 1.90 crore in the last IPL auction, doing the bulk of scoring.
There were also useful contributions from Dhruv Jurel (65) and Tilak Varma (42).
The visitors then returned to restrict South Africa to 198 for nine, continuing their preparation for the U-19 World Cup.
Left-arm medium-fast bowler Sushant Mishra was the pick of the bowlers, claiming four wickets for 48, while Ravi Bishnoi (2/29) and Kartik Tyagi (1/25), Atharva Ankolekar (1/29) and Tilak Varma (1/23) also chipped in.
Chasing 265, South Africa couldn’t get going with captain Bryce Parsons top-scoring with a 50-ball 57.
Andrew Louw hit a patient 85-ball 45 but none of the other batsmen could hang around for long.
India U-19s, who had come into the tournament after claiming a 2-1 win over South Africa in the Youth ODI series, will next face Zimbabwe on Sunday.
New Zealand is the fourth team in the competition.
India Under-19s 264/5 in 50 overs (Priyam Garg 110; Mondli Khumalo 4/53) beat
South Africa Under-19s 198 for 9 in 50 overs (Bryce Parsons 57; Sushant Mishra 4/48) by 66 runs.