The legal instrument establishing the Board was promulgated by the Act (No. 2 of 1978) of the Federal Military Government on 13th February, 1978. By August 1988, the Federal Executive Council amended Decree No. 2 of 1978. The amendments have since been codified into Decree No. 33 of 1989, which took effect from 7th December, 1989. Decree No. 2 of 1978 (amended by Decree No. 33 of 1989) empowered the Joint Admissions and Matriculation Board to:

(a) conduct Matriculation Examination for entry into all Universities, Polytechnics and Colleges of Education (by whatever name called) in Nigeria.

(b) appoint Examiners, Moderators, Invigilators, members of the Subject Panels and committees and other persons with respect to matriculation examinations and any other matters incidental thereto or connected therewith.

(c) place suitably qualified candidates in the tertiary institutions after having taken into account:

(i) the vacancies available in each tertiary institution;
(ii) the guidelines approved for each tertiary institution by its proprietors or other competent authorities (iii) the preference expressed or otherwise indicated by the candidates for certain tertiary institutions and courses (iv) such other matters as the Board may be directed by the Honorable Minister to consider or the Board itself may consider appropriate in the circumstances.

(d) collate and disseminate information on all matters relating to admissions into tertiary institutions or any other matter relevant to the discharge of functions of the board.

(e) carry out other activities as are necessary or expedient for the full discharge of all or any of the functions conferred on it under or pursuant to this Decree.