Mahmud Kanti Bello was elected as a Katsina senator in 2003 on the All Nigeria Peoples Party (ANPP) platform. In the 2004 local elections in his state, he protested strongly against electoral fraud which resulted in People's Democratic Party (PDP) candidates being returned. However, in 2007 he successfully ran for reelection on a PDP slate. By the middle of his second term in the senate, Mahmud Kanti Bello was among those senators who had not solely sponsored any private bills.
In a statement in January 2009, Mahmud Kanti Bello lashed out against governors who withhold local government funds, releasing them only at their convenience, including the governor of his state, Ibrahim Shema. In discussion over the proposed bill to control desertification, senator Bello strongly opposed Senator Grace Folashade Bent, chair of the Senate Committee on Environment and Ecology, who wanted to include other environmental issues in the bill.
In July 2009, South-South senators initiated a motion to demand dismissal of Minister of Petroleum, Dr. Rilwan Lukman and Mr. Mohammed Barkindo of the Nigerian National Petroleum Corporation on the basis of alleged anti-Niger Delta policies. Senator Bello opposed the motion on a point of order, without success. In 2009 Sen Kanti Bello was the NIgerian Amir-Hajj for the Nigerian pilgrims to Saudi Arabia. He broke a record where for the first time in over 20 years all the pilgrims left on time.