He served for 32 years as a civil servant in the Federal Ministry of Works and the Lagos State Government, retiring as a Director. He nurtured from infancy the department of sewage and drainage in Lagos State. In the ministry, he used Geiger Counter to monitor possible radioactive pollutants in Iju and Ogun rivers during the Civil war. 

Glowing testimony to his strong belief in Engineers being at the corridor of power is the fact that seven of the engineers trained by him ultimately became Permanent Secretaries in Lagos State. 

After retiring from the public service, he went into private practice of engineering, being involved in consulting, design and construction. Among the projects undertaken by Engr. Ajayi in private practice include sewage works for the University of Lagos, Lagos University Teaching Hospital, Lanlate Satellite Station, and the University of Ilorin. 

He was a COREN examiner for many years and was also prominent in the efforts that nurtured the Civil Division of the Nigerian Society of Engineers to the threshold of an Institution. 

He holds three Fellowships: American Society of Civil Engineers, Nigerian Society of Engineers and Nigerian Institution of Civil Engineers.