Mr. Justiniano started as an assistant in the Jose W. Diokno Law Office where he assisted the late Senator Jose W. Diokno in the trials of political detainees before the Military Commission created during the Martial Law years. He likewise worked with Sen. Diokno in handling some high profile criminal cases, such as the ambush of a governor in the Cordillera region allegedly by the latter’s political opponent whom they successfully defended, the prosecution of a police general and other police officials accused of mauling a civilian, and the defense of one of the accused in the so-called “Forbes Park” rape case. 

While he was with SyCip Salazar Hernandez & Gatmaitan Mr. Justiniano ventured into commercial litigation. He was assigned to try cases involving “white collar crimes” as well as enforcement of intellectual property rights through criminal prosecution. He was also tasked to handle cases involving contract disputes, as well as tort cases. He represented airline companies in damage suits filed by their passengers and was successful in fending off unreasonable claims, as well as in forging mutually-acceptable settlements of claims. 

Mr. Justiniano was tasked to act as one of the private prosecutors in the widely publicized murder of a socialite in an affluent residential subdivision in Makati City. In 2005, he defended four of the six U.S. Marines accused of raping a Filipina in Subic Base. At different stages of the case, he was successful in getting a dismissal or acquittal of all his clients. In 2011, he acted as one of the private prosecutors in the impeachment of the Ombudsman and in 2012, he was again engaged to be part of the prosecution team in the impeachment trial of the Chief Justice of the Supreme Court which resulted in the latter’s removal from office. 

In the Department of Justice, Mr. Justiniano was assigned as Undersecretary in charge of the investigation and prosecution of persons involved in the pork-barrel scam. Part of his job was to oversee the investigation being conducted by the National Bureau of Investigation of politicians, other public officials and private individuals who were involved in the criminal act. He was deputized by the Ombudsman to be a part of the team of special prosecutors handling these cases. He was also appointed as Head of the Internal Affairs Unit and Inter Agency Council Against Trafficking. Likewise, he was given the assignment of reviewing administrative cases against officials and personnel of the attached agencies of the Department of Justice. He also sat as ex-officio member of the Board of Directors of the following: Manila International Airport Authority, Mactan Cebu International Airport Authority, Civil Aviation Authority of the Philippines, and the Housing and Land Use Regulatory Board.