Lidoda Duvha enjoyed the best of the opening exchanges, with Alpheus Kekana shooting just over in the second minute of the game.
Myron Shongwe should have handed the hosts the lead in the 24th minute when he rose highest to meet a cross by Christopher Netshidzivhe, but the striker’s effort flew straight at Celtic stopper Postnett Omony.
The visitors went ahead in the 41st minute, against the run of play, when Mungomeni turned a shot by Petrus Mahlatsi into his own net.
The score remained 1-0 at the break.
Celtic had a better start to the second stanza, with Gary Goldstone sending an effort over the bar from long range in the 51st minute.
Jabulani Maluleke should have pulled Leopards level in the 53rd minute after he collected a good cross by Shongwe, but the midfielder sent his effort wide from just inside the area.
Maluleke wasted another chance in the 78th minute when he sent his header, from a promising position, just over the bar following a good cross by Mungomeni.
Celtic could only muster one shot at the Leopards goal, with substitute Lebohang Kukame sending his effort straight at keeper Ephraim Tjihonge, and with that went the final chance of the game.