He graduated his Bachelor of Laws summa cum laude from the University of Santo Tomas in 1924, then placed first in the bar examinations that same year.
Starting out as a private practitioner, he then worked at the Office of the Solicitor General, from which he was subsequently appointed as a judge, then a Court of Appeals Justice before being appointed as Supreme Court Associate Justice in 1954.
Concepcion, one of the leaders of the Civil Liberties Union and a Constitutional expert in his own right, advocated the promotion and protection of civil and individual liberties. His dedication to the Rule of Law was his hallmark.
As Chief Justice, he paved the way of accepting a more liberal approach regarding the individual rights and liberties, whether personal or civil. Said acceptance was shown in the admissibility of evidence, in which the Supreme Court, under his helm, declared that illegally seized evidence is not admissible(Though some jurisdictions, including the U.S., made inadmissible evidence illegally seized objects earlier on, it was only in 1967 that such evidence in Philippine jurisdiction was deemed unacceptable).
He is a good administrator of the Court, and followed a systematic approach in the assignment and organization of the casework.
Concepcion wrote the decision in the Ratification Cases which upheld the 1973 Constitution. In the said decision, he wrote the summary of facts, then his own opinion of the case (which he said that the 1973 Constitution has not been properly ratified according to law), then proceeded to make the summary of votes.
The court was divided on the issues raised in the petition: but when the question of whether the petitioners in the cases are entitled to relief, Concepcion, together with three others answered ‘Yes’, while six other members denied the relief being sought, thus upholding the 1973 Constitution and made legitimate the rule of Marcos.
When the decision came out to the public, the last sentence of Concepcion's ponencia contained the following last words:
"This being the vote of the majority, there is no further judicial obstacle to the new Constitution being considered in force and effect".
It is disputed as to whether or not Concepcion placed the said sentence intentionally, or that someone intercalated the said words after he signed the decision.
After leaving the Supreme Court, he became one of the advocates against the ensuing Marcos regime. Together with former Justice and best friend, J.B.L. Reyes, they encounted cases with questioned the validity of government acts, especially in the wake of suppressed civil and individual liberties at that time.
He also found time to return to his alma mater, UST, where he briefly served as dean of its Faculty of Civil Law.
After the toppling of Marcos from power, Concepcion was appointed as one of the commissioners tasked to draft the 1987 Philippine Constitution. A one of its members, it is responsible for crafting the contents regarding civil liberties, as well as it added as a provision in the Executive Powers of the President, a clause limiting the effects of martial law with respect to the writ of habeas corpus, based on one of the decided cases by the Supreme Court in which he wrote. He is also responsible for strengthening the independence of the judiciary, which was clearly abused by the Marcos regime.
People v. Hernandez (99 Phil. Reports 515, 1956): the Supreme Court, through then Associate Justice Concepcion, ruled that rebellion cannot be complexed with other crimes, such as murder and arson. Rebellion in itself would include and absorb the said crimes, thus granting the accused his right to bail.
Stonehill v. Diokno (G.R. No. L-19550, June 19, 1967; 20 SCRA 383): It was ruled that the articles that were seized illegally by the government cannot be used as admissible evidence, thus adopting the fruit of the poisonous tree doctrine in Philippine jurisdiction. It abrogated the principle established in an earlier case (Moncado v. People’s Court, 80 Phil. Reports 1). During the time between the Moncado and Stonehill decisions, Concepcion dissented in every case which would uphold the admissibility illegally seized evidence, citing the U.S. cases of Weeks v. U.S.(232 U.S. 383, 1920) and Elkins v. U.S.(364 U.S. 206, 1960). Said case also established the definition of probable cause, which requires that allegations should be specific in the description of the offense or crime committed, as well as to the evidence subject of the search warrant.
Lansang v. Garcia (G.R. No. L-33964, December 11, 1971; 42 SCRA 448): The Supreme Court, through Concepcion, while it upheld the suspension of the writ of habeas corpus by Marcos, declared that the Judiciary has the authority to inquire to the factual basis of such suspension, and that the suspension is to be annulled if no legal ground would be established. This doctrine is now established by the 1987 Philippine Constitution as it is included in one of its provisions.
Javellana v. Executive Secretary (G.R. No. L-36142, March 31, 1973; 50 SCRA 30): Concepcion’s last ponencia, he formally delivered the summary of votes in upholding the 1973 Philippine Constitution, but delivered in his own opinion his disapproval that the said Constitution was in effect and ratified properly by the Filipino people.