Kammala jumla mai zuwa: A baya a Congo-Brazzaville, Mondjo ya yi aiki a matsayin jami'in soja, kuma ya kasance Daraktan Darasi da Nazarin a Makarantar Sojojin Marien Ngouabi a Brazzaville daga 1987 zuwa 1993. Ya goyi bayan jagoran 'yan tawaye Denis Sassou Nguesso a lokacin yakin basasa.
Yuni-Oktoba 1997 yakin basasa; yaƙin ya ƙare tare da Sassou Nguesso ya sake samun iko kuma ya kori Shugaba Pascal Lissouba. A watan Disamba na shekarar 1997, an nada Mondjo a matsayin Kwamandan Yankin Soja na 1, wanda ya hada da babban birnin tattalin arzikin kasar, Pointe-Noire; ya kasance a wannan mukamin na tsawon shekaru biyar. An nada shi a matsayin Shugaban Ma'aikatan Sojojin Kwango a ranar 20 ga Disamba, 2002, kuma an daukaka shi zuwa mukamin Manjo Janar ("général de division").