García Sayán was born in Brooklyn, New York, the youngest son of Dr. Enrique García-Sayán, a former Foreign Affairs Minister of Perú who, in 1946, was associated, along with President José Luis Bustamante y Rivero, with launching the so-called "200 Nautical Miles (370.4 km) Territorial Doctrine", currently being adhered to and claimed by Benin, Congo, Ecuador, El Salvador, Liberia, Perú and Somalia.

[5] From 1991 to 1992 he joined the UN Negotiating Team appointed by Secretary General Boutros Boutros-Ghali to mediate into the Guatemalan Peace Process between the Government and the URNG.

Following the political collapse[7] of the Fujimori's regime a Transitional Government was formed and Garcia-Sayan was called by President Valentin Paniagua to serve as Minister of Justice from 2000 to 2001.

[8] On January 27, 2017, United Nations Secretary-General António Guterres designated him a member of the Selection Mechanism for the Special Jurisdiction of Peace, established in Colombia in 2016.

[10] He is also currently serving as an advisory board member of the Global Judicial Integrity Network at the United Nations Office on Drugs and Crime (UNODC).