On his return from England, Udoji joined the Colonial Administrative Service and was made an assistant district officer in Ado Ekiti before being posted as district officer for Egbado. As colonial administrative officer he had a distinguished service in Ondo and Abeokuta provinces of the country. In 1954, he was transferred to the Eastern region and made the permanent secretary in the ministries of Health, Commerce, Finance and Establishments. In 1959, Udoji's exemplary dedication to duty, his remarkable intellectual capacity and his integrity made him rise up to the level of Head of the Region’s civil service, Chief Secretary to the Premier of the Eastern Region, Dr Michael Okpara, as well as Secretary to the Executive Council. He remained in that position until the 1966 Nigerian coup d'état. Following the coup he went into private legal practice (1966–68) and served as a Ford Foundation Consultant in Administration and Management (1968–72).