By 1997, Mannah got employed at NLNG, Bonny as a security consultant and by 1999, he got elected as an NLNG Workers’ Union Welfare Officer where he led the longest workers’ strike action lasting for one month and six days that attracted the attention of Adams Oshiomole, the then NLC President.
After his job at the NLNG, Bonny in 2002, he worked at Daewoo, an oil servicing company in Ahoada before doing business as a civil contractor with government agencies in 2003.
In 2011, after losing an election, Mannah ventured into international fashion retail business, after which in the same year, he took up a job with Felix Obuah as a Chief Surveillance Officer.
In 2015, he campaigned to represent Ahoada East I at the Rivers State House of Assembly where he won after a supplementary election.
Manna established an NGO, Martyns-Mannah and Friends Foundation, with a goal of poverty alleviation, especially in Ahoada East local government area.