Leonen is sometimes called "The Great Dissenter" for his liberal and human rights-oriented decisions, often in opposition to the more conservative justices who were close to Rodrigo Duterte.

[6] In his senior year, Leonen joined the free legal assistance group in 1988 and remained a member until his appointment to the Supreme Court in 2012.

[9] He also directed the college's clinical legal education program and has lectured and been a resource speaker at national and international forums in Hong Kong, Thailand, Malaysia, Indonesia, Japan, Spain, Netherlands, Australia, Estonia, and the United Kingdom.

In July 2010, President Benigno Aquino III named Leonen as the Philippine government's chief negotiator with the Moro Islamic Liberation Front.

[12] Under his leadership, the government successfully created a framework agreement with the MILF to establish the Bangsamoro political entity, replacing the Autonomous Region for Muslim Mindanao.

[14] He stated, "A member of the House of Representatives or a Senator is not an automated teller machine from which the public can withdraw funds for sundry private purposes.

"[15] In civil law, Leonen penned many notable decisions, including Tan-Andal v. Andal, which removed the requirement of medical or expert witnesses for declaring a marriage null due to psychological incapacity under the Family Code.

The incapacity to fulfill essential marital obligations need only be manifest specifically toward that spouse to declare the marriage void ab initio.

[16] Even if the Chief Justice has failed our expectations, quo warranto, as a process to oust an impeachable officer and a sitting member of the Supreme Court, is a legal abomination.

It creates a precedent that gravely diminishes judicial independence and threatens the ability of this Court to assert the fundamental rights of our people.

[18]: @34:00 Among Leonen's notable dissents is his argument in Disini v. Secretary of Justice that criminal libel and cyberlibel are unconstitutional vestiges of American and Spanish colonialism.

"[17] In Lagman vs. Pimentel III, a case addressing the legality of extending martial law and suspending the privilege of the writ of habeas corpus in Mindanao, Leonen explained that Congress had abused its discretion because there was no proper presentation of facts, no examination of the allegations by the military, and no ascertainment of why a longer extension was needed despite the continued declaration of military victory.

[22] The Supreme Court En Banc approved the Code of Professional Responsibility and Accountability (CPRA) on April 11, 2023, after a nationwide caravan with members of the legal community and the public.

So help me God.Proposed Revised Lawyer's Oath JUSTICE MARVIC MARIO VICTOR F. LEONENI, (name), do solemnly swear (affirm) that I accept the honor, privilege, duty and responsibility [duties and responsibility] of practicing law in the Philippines as an Officer of the Court in the interest of our people.

for affirmations)On May 25, 2024, Leonen announced at his Integrated Bar of the Philippines Caloocan-Malabon-Navotas Chapter lecture at the Manila Hotel that the Supreme Court of the Philippines was drafting the "Writ of Kalayaan," a writ and constitutional remedy to address prison overcrowding of detention suspects (PDLs) and protect their human and legal rights.

[26] However, on October 19, 2023, Bureau of Corrections's Gregorio Catapang Jr. contradicted the legal remedy recommended by the SC’s judicial panel "Committee on Human Rights and International Humanitarian Law" on December 19, 2022, citing Republic Act 10575 (Bureau of Corrections Act of 2013), RA 11928, and the Department of Justice's 5-year Development and Modernization Plan 2023-2028.

[27] In December 2020, Edwin Cordevilla, claiming to be the secretary general of the Filipino League of Advocates for Good Government and represented by lawyer Larry Gadon, filed an impeachment complaint against AJ Marvic Leonen.

The complaint alleged that SAJ Leonen had been “incompetent and negligent” for failing to resolve 37 cases within the required period and lacked integrity for not filing his SALN for 15 years.

Leonen was accused of culpable violation of the Constitution for failing to resolve cases in a timely manner and betrayal of public trust for not filing his SALN.

On December 10, 2020, the UP College Of Law released a statement dismissing the impeachment complaint,[29] stating that Leonen “has expressed consistently, in his decisions and dissents, a commitment to academic rigor, principled discourse, creative expression, and courageous authenticity” and that the act was “a latest assault to send our nation further down the dark road, especially while a global pandemic still hangs over all our heads and threatens to bring our society into despair and destitution.” Senator Risa Hontiveros also called the impeachment unnecessary and counterproductive, deeming it a “distraction that will only drag lawmakers and the public into a pointless political fiasco” amid the pandemic.

And at that time we were only starting to discover that Martial Law was not what it was meant to be; that there were there was pervasive poverty all around; and human rights violations were happening.

I entered the UP College of Law in 1983 in the following year- Ninoy Aquino was shot, Lean Alejandro was assassinated and we were on the streets walking and asking that the dictator to come down.

Then I remember Constitutional Law Article 3 Section 2, "The right of the people to be secure in their persons, places, papers and effects against unreasonable searches and seizures shall be inviolable.

We scoured the country, work with Indigenous' Peoples Groups, went up mountains- Mount Apo several times- six times, I think, with Joey Ayala.

This was the first and environmental impact statement assessed by the DENR and we thought that it was high time because there was Section 16 of Article 2 of The Constitution that the rhythm of harmony and nature required that the Court extended its judicial review more strictly into administrative cases coming from the DENR, so that not only substantial evidence could be examined but they would look up the use of science in terms of the environmental impact statement.

The first largest fully foreign owned mining concession in this country was unconstitutional, null and void ab initio.

You will be caught for color coding, you will be arrested for some kind of an offense, including singing the National Anthem in the wrong way [crowd's laughter].

I've experienced in this; [crowd's laughter], the only way that I, one of the things that you can separate from your spouse is to actually prove the condition called "psychological incapacity", a concept of only one religious sect within our country and not of all, but still this law exists.

[38] Leonen has a following among law students and the youth, often posting jokes on love and beliefs based on his favorite books on Twitter and in graduation speeches and socio-political forums.

He has a Cordilleran-inspired tattoo depicting a lizard around his right wrist, with designs featuring combinations of snake and centipede symbolisms (called "tinulipao", "tab-whad", "inang-oo" and "gayaman").