He came to Sierra Leone in 1980 and played for East End Lions and won the championship in 1980 and 1985. Mensah gained Sierra Leonean citizenship in 1987.
Mensah made his international debut for Sierra Leone in the 1988 African Cup of Nations qualification competition in a 2-0 victory over Nigeria in July 1987. He made 6 caps before moving to the Netherlands in 1989. There he had a trial at Sparta Rotterdam and played in the amateur ranks ending at ASV Arsenal. He got his UEFA B-licence in the Netherlands and worked as a youth coach. He is currently head coach of amateur-side ASV Arsenal in Amsterdam.
Mensah was appointed by the Ministry of Sport in September 2014, replacing the Northern Irish manager Johnny McKinstry in the role of manager of the Sierra Leone national football team for the remaining 2015 Africa Cup of Nations qualification matches. He was replaced by John Ajina Sesay, who was appointed by the Sierra Leone Football Association in October 2014.
- "Atto Mensah appointed Sierra Leone coach". BBC Sport. 18 September 2014. Retrieved 18 September 2014.
- Bondscoach van Sierra Leone in tijden van ebola, Volkskrant, 8 October 2014
- "Voetballen om ebola te vergeten", nos.nl, 3 October 2014
- Mohamed Fajah Barrie (11 October 2014). "2015 Nations Cup: Sesay replaces Mensah as Sierra Leone coach". BBC Sport. Retrieved 11 October 2014.