Leinster, the defending United Rugby Championship champions, were defeated by the Lions with a final score of 27-26 at Ellis Park on Saturday.
Fintan Gunne delivered an outstanding performance for his side, scoring three tries in the first half alone.
The Irish province took the lead early when Gunne broke through the Lions defence to cross for the opening point, followed by Erich Cronje's response for the hosts.
Chris Smith converted Cronje's try and later added a penalty kick to extend Johannesburg's advantage during the match.
Gunne responded with his second try after Josh Ericson broke through the defensive line and offloaded him over the try line.
Cronje scored again to restore the home side's lead, while Gabriel missed a crucial penalty attempt for Leinster.
Gunne completed his hat-trick by grounding the ball after running over the line, putting Leo Cullen's team ahead 19-17 at half-time.
A penalty kick from Smith moved the Lions back into the game by a single point shortly before the break.
The match saw disciplinary action when Dylan McNeice received a yellow card for Leinster in the final ten minutes of play.
Leinster continued their tour with an away fixture against the Sharks scheduled for next Saturday.
The Lions will host the Ospreys in their subsequent home game.
Caspar Gabriel kicked two extra points for Leinster, while Kelly Mpeku scored late to give the hosts a try shortly before full-time.
Stephen Smyth converted Mpeku's try to seal the victory for the Lions.
Harry Byrne added two points for Leinster after a late score by Stephen Smyth following a sin-bin dismissal of replacement SJ Kotze for the home side.