He enrolled in the graduate program in constitutional law offered by Pontifical Catholic University of Peru in 1993, but did not complete the degree.

A prominent member of the Peruvian Aprista Party, Del Castillo was first elected to public office as a Councilman of the District of Barranco in 1981.

During his short term in Congress, he defended APRA leader Alan García from the accusations against him involving corruption and unjust enrichment.

For the general elections of 2000, he was a candidate for the 1st Vice Presidency of the Republic on the presidential roster of Abel Salinas Izaguirre for the APRA, however the candidacy was not successful and he was in the 4th place of the preferences.

Del Castillo was sworn in as García's Prime Minister on July 28, 2006, swearing for Víctor Raúl Haya de la Torre and "APRA martyrs blood".

After the "Petroaudios" scandal revealed Del Castillo's involvement, he tendered his resignation with the whole cabinet on October 10, 2008, to President Alan García, marking his political downfall.

In the 2016 general election, Del Castillo was reelected for a fifth term in Congress, returning after 5 years of absence under the Popular Alliance.