The 21-year-old opened the scoring for his side just ahead of the break, while second-half goals from Tommy Conway and Nahki Wells added daylight to the score for Bristol City.
Jay Bird scored the only consolation for visitors MK Dons in the last minute of the match.
The victory came with a wonderful display from both Saikou Janneh and his team as they grabbed a first pre-season victory.
Gunjur-born player, Janneh signed for Bristol City in 2018 from Clevedon Town but was previously sent on loan at Toryau United and Newport Country respectively.
He was a center of focus to some Premier League clubs after putting an impressive display last season with Newport County in the English Fourth Division League.
Saikou Janneh and Bristol City remain unbeaten in pre-season and play again on Tuesday, where the players will be put to the test with matches against Exeter City and Plymouth Argyle.