He is the son of the late Arnoldo Castillo, a leading figure in the Catamarca UCR.

Castillo was born in San Fernando del Valle de Catamarca.

Castillo served as a provincial deputy (legislator) in Catamarca (1985–89), and was UCR-MPC caucus leader.

Castillo, who had taken his oath as a legislator without mentioning God in two previous occasions, imposed mandatory religious education of a Roman Catholic nature in Catamarca schools; protest from the province's sizable Jewish community and others led to the policy's rescission in 2001, however.

His reelection campaign, however, was followed by a challenge of the results amid accusations of vote buying and collusion with Governor Eduardo Brizuela del Moral to that effect; his personal estate, which includes at least a dozen properties in the province, has also come under scrutiny.