Entering private business, he became Managing Director of Kaduna Investments Company, and served on the Board of New Africa Merchant Bank, Bank of the North, the Nigeria Stock Exchange (Kaduna), Cement Company of Northern Nigeria and Katsina Oil Mills.
[2] After taking his seat in the Senate in June 1999, Mamman was appointed to committees on Ethics, Security & Intelligence, Environment, Commerce, Finance & Appropriation (chairman) and National Planning.
[5] However, in October 2002, he sued New Nigerian Newspapers for publishing a story that alleged he resigned because he had been found guilty of wrongdoing by the Kuta panel.
[7] After the International Court of Justice decision in October 2002 that the Bakassi peninsula belonged to Cameroun, Mamman urged caution to avoid the risk of war.
[8] Due to friction with President Olusegun Obasanjo and his supporter in the Senate, Mamman left the PDP for the All Nigeria People's Party (ANPP) before the 2003 elections.