Mondeh was commissioned into the Sierra Leone Army in 1991 as a Second Lieutenant. During Sierra Leone's 10-year civil war, he helped plan and execute the recapture of Pendembu, Kailahun, Kono, Mile 38, Nomo Fayama and Sierra Rutile. He was then sent on a Senior Division Staff Course in Nigeria in July 1996, and was invited to attend the Nigerian War College the following year, where in addition to his military qualification, he earned a master's degree in Strategic Studies. Mondeh was also awarded the 'Best International Student'. He pursued a master's degree program in Strategic Studies at the University of Ibadan, Nigeria between September 1998 and July 1999.
