During the 1970s, Cortes was also active in the Integrated Bar of the Philippines, chairing its Committee on Professional Responsibility, Discipline and Disbarment from 1977 to 1984.

Cortes was named Associate Justice of the Supreme Court on February 1, 1987, by President Corazon Aquino.

Her opinion for the Court in Valmonte v. Belmonte (1989) set forth the parameters for the constitutional right to information, as guaranteed under Article III, Sec.

Reflecting views she had previously published before she was appointed to the Court, Cortes wrote that the President possessed "residual unstated powers" beyond those granted by the Constitution "to protect the general welfare"; and that the exercise of these powers justified the presidential ban against the return to the Philippines of the deposed Ferdinand Marcos.

Associate Justice Robert H. Jackson in his concurring opinion in Youngstown Sheet & Tube Co. v. Sawyer, 343 U.S. 579 (1952), remain the subject of academic debate.