Bello worked in private practice in the 1970s and 1980s and became active in several civic and human rights groups during the Marcos dictatorship such as the Free Legal Assistance Group (FLAG), the Justice for Aquino, Justice for All (JAJA) Movement, the Coalition of Organizations For The Restoration of Democracy and the Coalition For The Protection of Workers’ Rights. He was appointed by then PresidentCorazon Aquino as an undersecretary of the Department of Justice from 1986 to 1991. Bello served as Justice Secretary from 1991 to February 1992 when he resigned to run for the Senate as a candidate of Fidel V. Ramos' Lakas-CMD party in the May 1992 elections, which he lost. He was then appointed to several government positions by President Ramos such as the government negotiating panel during peace talks with the CPP-NPA-NDF before running again in 1995 for governor of Isabela but lost to Benjamin Dy. He was then reappointed by President Ramos as Solicitor General from September 23, 1996, until February 3, 1998[5] when he was reappointed Secretary of Justice. In June of that year he was also concurrently reappointed as Solicitor General which he occupied until the end of Ramos' presidency on June 30.
He was the Chairman of the Government Negotiating Panel for Talks with the CPP-NPA-NDF from January 2001 to August 2004. He served as president and CEO of PNOC Development and Management Corporation from November 2004 to December 2005. From January 2006 to December 2006 he was the General Manager and CEO of the Philippine Reclamation Authority. He was a presidential adviser for New Government Centers from July 2007 to July 2008. He served as Cabinet Secretary during the presidency of Gloria Macapagal Arroyo.[10][11]