[2] Kumar is married to Geetha, a gynecologist in Sree Uthradam Thirunal Hospital, Pattom, Thiruvananthapuram.

G. Mohan Kumar is a graduate (BSc) and a postgraduate (MSc) in Chemistry from University College.

[3][4][5] During the course of his career, Kumar served in various key positions for both the Union Government and the Government of Odisha, such as Principal Secretary (Fisheries and Animal Resources Development), Commissioner of Commercial Taxes, Chairman and Managing Director of Odisha State Road Transport Corporation (OSRTC), Inspector General of Registrations, Managing Director of Odisha Small Industries Corporation (OSIC), Secretary to the Governor of Odisha, Chief Executive of Odisha Renewable Energy Development Authority, Managing Director of Odisha State Seeds Corporation (OSSC), and as the District Magistrate and Collector of Sambalpur district in the Odisha Government,[3][4] and as the Union Defence Secretary, Union Defence Production Secretary, Union Steel Secretary, Special Secretary in the Ministry of Water Resources and as the Chairman of the Marine Products Export Development Authority in the Government of India.

[3][4] Kumar was appointed as the Union Steel Secretary by the Appointments Committee of the Cabinet (ACC), he assumed office on 3 September 2013,[3] and demitted it on 29 August 2014.

Kumar was appointed as the Union Defence Secretary by the Appointments Committee of the Cabinet (ACC) in May 2015,[6][7][8][9] he assumed the office of Defence Secretary on 25 May 2015,[3][4][10][11] and demitted it and simultaneously superannuated from service on 24 May 2017.