Throughout its existence, the department's functions are to enforce the law and to curb criminality and to guarantee the external and internal security of the country.

As such, the DND deals also with criminal elements because the Philippine Constabulary or PC, then a major branch of the Armed Forces that has responsibility in enforcing the law and maintaining peace and order was under its supervision, aside from other AFP major services (Army, Navy, Air Force) which are busy dealing with fighting insurgents like the Communists and Muslim secessionists.

In effect, NISA also served as the secret police force of Marcos, and now became the most powerful agency, and the MND's status dwindled.

At the end of Marcos regime on 1986, the DND continued to exercise the powers to deal with criminality and internal plus external defense of the country until 1991 when then President Corazon Aquino signed Republic Act No.

6975 passing the functions to enforce all criminal laws from DND to the Department of the Interior and Local Government or DILG.