On his return to Nigeria in 1968. Engr. Adedeji worked with Eastern Nigeria Water Corporation (1968-1971) and the Lagos State Development and Property Corporation (1971-1973) He left the Government Service as a Deputy Chief Engineer in 1973 to join Sshwed Associates, Consulting Civil/Structural Engineers, as a founding principal partner, till 1983 when he founded his own Civil/Structural Engineering Consultancy firm - Segun Adedeji Associates, where he is the principal consultant to date. He has been involved in over 200 civil and structural Engineering projects all over the country. He served as a Member of the Presidential Projects Assessment Committee, inaugurated by President Dr. Goodluck Ebele Jonathan, in March 2010.
Engr. Adedeji`s service to the engineering profession in Nigeria spans over three decades, serving in most committees of the Nigerian Society of Engineers (NSE) and the Council for the Regulation of Engineering in Nigeria (COREN). He was at the vanguard of the transformation of the Civil Division of the NSE to the Nigerian Institution of Civil Engineers, becoming the pioneer Chairman of the Institution 2002-2003. He was the Secretary General of the NSE 1982-1984, and the Registrar of COREN 1988-1990.
He received the Nigerian Society of Engineers NSE Award, the Council for the Regulation of Engineering COREN Meritorious Service and the Nigerian Institution of Civil Engineers Merit Award. He has published several publications.