He was elected Delegate to the National Constitution Conference (1994–1995). During the transition planned by General Sani Abacha, Simeon Ajibola ran for senate on the United Nigeria Congress Party (UNPP) ticket. Although regarded as an underdog, he went ahead to win. However, the process was cancelled with Abacha's death in June 1998. After the return to democracy in 1999, Ajibola was Federal Commissioner, Revenue Mobilization, Allocation & Fiscal Commission (2001–2002).
Ajibola ran for Senator for the Kwara South constituency in April 2003 but was defeated by Suleiman Makanjuola Ajadi of the All Nigeria Peoples Party (ANPP). However, in June 2004 Ajadi's victory was overturned by the Election Petition Tribunal and Ajibola was declared elected. He was reelected in 2007 and was appointed to committees on Water Resources, Upstream Petroleum Resources, Security & Intelligence and Agriculture. In a mid-term evaluation of Senators in May 2009, ThisDay said he had not sponsored any bills in the previous year, had limited involvement with debate in plenary, but was effective in committee work.
Ajibola ran again for the Kwara South Senate seat in the 9 April 2011 election, and was returned for a third term.
In December 2014, Senator Ajibola defeated the seemingly more popular candidates Dele Belgore and Gbemisola Saraki to clinch the PDP primaries to become the party flag bearer running for Governor of Kwara State in the 2015 elections.