Adeseun was elected on a non-party basis as Executive Chairman of the Surulere Local Government, Iresaadu, Oyo State, holding office from March 1996 to January 1997, and was reelected to this position on a party basis, holding office until July 1998. He was the founding State Chairman of the All People's Party in Oyo State. From October 1999 to February 2003 he was Chairman of the Board of Directors, Oyo State Transport Company.
In April 2003 Adeseun was elected a member of the Federal House of Representatives. He was appointed Chairman of the Appropriations Sub Committees on Power & Steel, and Science & Technology and was also a member of House Committees on Appropriations, Education, Judiciary, Information and Gas Resources. Adeseun was reelected to the House of Representatives in April 2007 for the Ogo-Oluwa/Surulere Oluyole constituency, running on the People's Democratic Party platform. He was appointed Chairman of the South West Parliamentary Caucus, Chairman of the House Committee on Appropriations, and member of House Committees on Pension Matters, Rural Development, Sports and National Planning. Adeseun defected to the Action Congress of Nigeria party in 2010. In the 9 April 2011 election for the Oyo Central Senate seat, Adeseun ran on the platform, winning 105,975 votes, ahead of Luqman O. Ilaka of the Accord party with 92,544 votes and the People's Democratic Party candidate with 78,643 votes. In February 2011 Adeseun said the Federal government had only recorded about 40% of the capital budget being implemented, and this was only about 30% of the total national budget. He said lack of capital expenditure was not a good omen for Nigeria. Senate President David Mark agreed, saying "No nation desirous of meaningful development can afford such a disproportionate allocation of its financial resources between consumption and investment". Adeseun noted a wide discrepancy between claims of implementation rate between the legislature and the executive. Speaking in March 2011, Adeseun said that the Appropriation Bill 2011 had been passed out of the national interest, citing intense pressure from the Executive, despite the initial refusal by the House since some agencies had failed to present their budgets.
Senate
Adeseun was elected to the Senate in 2011, assuming his seat on 29 May 2011. He is the Chairman of the Capital Markets Committee and a member of the Solid Minerals Committee. In October 2012, as the Capital Markets chairman, Adeseun urged President Goodluck Jonathan to remove Arunma Oteh from her position as Director-General of the Security and Exchange Commission, calling his choice to reinstate her "a flagrant disregard of the resolution of the House of Representatives."