Leonen joined the law faculty in 1989 as professorial lecturer in Philippine Indigenous Law.
He became assistant professor during the term of Dean Pacifico Agabin and started to do work as an academic administrator under the term of Dean Merlin M. Magallona.
In 2000, he joined the University of the Philippines System administration as University General Counsel.
In March 2005, he became the first Vice President for Legal Affairs of the UP System.
Leonen has taught 20 different subjects in the law school.
He also served as director of the clinical legal education program of the college.
His teaching competence is not only acknowledged in the CL but also in other institutions like Miriam College and the Philippine Judicial Academy, to cite a few.
He has extensively lectured and acted as resource speaker in other national and international fora such as those conducted in Hong Kong, Thailand, Malaysia, Indonesia, Japan, Spain, Netherlands, Australia, Estonia and the United Kingdom.

In 2008, he was selected as the Dean of the University of the Philippines College of Law at UP Diliman by the Board of Regents of the UP System.
He served as Dean until the appointment of Danilo L. Concepcion in June 2011.

Leonen has also provided legal commentary for television networks such as ABS-CBN and GMA Network during the coverage of such events as the 2001 impeachment trial of President Joseph Estrada.
Leonen joined the law faculty in 1989 as professorial lecturer in Philippine Indigenous Law. He became assistant professor during the term of Dean Pacifico Agabin and started to do work as an academic administrator under the term of Dean Merlin M. Magallona. In 2000, he joined the University of the Philippines System administration as University General Counsel. In March 2005, he became the first Vice President for Legal Affairs of the UP System. Leonen has taught 20 different subjects in the law school. He also served as director of the clinical legal education program of the college. His teaching competence is not only acknowledged in the CL but also in other institutions like Miriam College and the Philippine Judicial Academy, to cite a few. He has extensively lectured and acted as resource speaker in other national and international fora such as those conducted in Hong Kong, Thailand, Malaysia, Indonesia, Japan, Spain, Netherlands, Australia, Estonia and the United Kingdom.
In 2008, he was selected as the Dean of the University of the Philippines College of Law at UP Diliman by the Board of Regents of the UP System. He served as Dean until the appointment of Danilo L. Concepcion in June 2011.
Leonen has also provided legal commentary for television networks such as ABS-CBN and GMA Network during the coverage of such events as the 2001 impeachment trial of President Joseph Estrada.
Marvic Leonen