After privately practicing law for 19 years, in 1990 Governor Rafael Hernández Colón appointed Andreu to fill a vacancy as Associate Justice in the Supreme Court of Puerto Rico.
[1] Chief Justice Andreu had a brief dispute with Governor Pedro Rosselló in 1997 when Rosselló decided to have Associate Justice Baltasar Corrada del Río deliver the oath of office to him during his second inauguration.
After eleven years at the helm of the court, Andreu decided to resign in 2003 during the governorship of Sila Calderón.
Calderón initially appointed Secretary of State Ferdinand Mercado to succeed Andreu but he was rejected by the Senate.
Eventually, Andreu was replaced by his colleague Associate Justice Miriam Naveira.