Ronnie Dimestico came to Port Charles with fellow police officer and childhood friend Dante Falconeri to bring down organized crime. Ronnie was Dante's superior within the police force although they both went undercover in the Zacchara organization. Soon after Dante infiltrated the Corinthos organization and Ronnie met with him at a laundromat to exchange information. Dante's mother Olivia interrupted a meeting between Ronnie and Dante and she begged Ronnie to reassign Dante to another case but he refused. He explained that Dante had gone further in the Corinthos organization than any officer before him ever had and he was not willing to jeopardize their case.
Dante and Ronnie disagreed about how to proceed in building a case against Sonny Corinthos after Sonny's wife Claudia discovered Dante's true identity. Ronnie ordered Dante off the case but Dante refused to obey the order. An influential rival of Sonny's resolved the issue by pressuring their superiors to keep Dante on the case. Ronnie remained concerned over Dante's involvement and worried that Dante was developing compassion for Sonny. He concerns grew after Dante's girlfriend Lulu Spencer also learned that Dante was an undercover police officer.
The worries that Ronnie had regarding Dante's undercover role were eased after Sonny's wife was murdered. Dante gathered evidence that could send Sonny to prison and Ronnie believed they were close to sending Sonny away which would safely extract Dante from his undercover role.
Around the same time, someone from Ronnie's past resurfaced in Port Charles. The person was an artist named Franco and he left his police booking identification as a graffiti tag around town. Franco was a suspect in the murder of mobster Joey Limbo and drew attention from the police. Fellow police officer Lucky Spencer questioned Ronnie's connection to Franco after Lucky realized that Ronnie had booked Franco several years ago. Ronnie claimed that he did not remember Franco.
Privately Ronnie was approached by Franco and Ronnie admitted that he had covered for Franco so he would not be arrested for the murder of Joey Limbo. A conversation between the two revealed that several years earlier Franco had taken the fall for a crime that Ronnie had committed so his acceptance into the police academy would not be compromised. Franco issued Ronnie a warning and shortly after Ronnie was hit by a car. He believed Franco was behind his accident.
While Ronnie recovered, he and Dante were offered positions within the Port Charles police department. Dante's undercover work was over after Sonny learned his real identity and both Dante and Sonny learned that they were related. Sonny was Dante's father and Ronnie questioned whether Dante would be able to send Sonny to prison after Dante was shot presumably by Sonny but he refused to press charges against him.
Ronnie and Dante continued to disagree about Dante's loyalties to Sonny as the investigation into the death of Claudia Zacchara continued. Ronnie believed that Sonny's children had information about the murder and tried to question them. Dante defended his new brothers and Ronnie believed it was clear that Dante's integrity had been compromised due to his relationship with Sonny.
Eventually Claudia Zacchara's killer was revealed not to be Sonny and Ronnie and Dante repaired their friendship as they continued their work to bring down organized crime. Ronnie remained relentless in his pursuit of Sonny and he remained willing to use Sonny's family to accomplish his goals although Dante disproved of his methods and tried to thwart Ronnie when possible.
Despite Ronnie's best efforts he was unable to gather any evidence against Sonny until he manufactured some. He came across Sonny's rival Johnny Zacchara bleeding from a gunshot wound that Johnny claimed had come from Sonny. Ronnie was the first person on the scene and he disposed of Johnny's gun so it appeared that Johnny was unarmed when Sonny shot him. Despite Ronnie's interference, Sonny was not prosecuted.
Ronnie and Dante each continued their work for the police department and organized crime remained their focus. However, a case involving females that were being attacked drew their attention especially after Dante's new partner Delores Padilla was attacked. Ronnie and Dante suspected that the attacker might be Officer Padilla's husband Eddie and they believed their suspicions were correct after they found pictures of the women that had been attacked in Eddie's closet.
Delores insisted her husband was innocent and Dante's wife Lulu suspected that Ronnie had framed Eddie by planting the evidence when he conducted the search. She believed Ronnie was behind the attacks on the dancers. Ronnie caught Lulu looking him up on the police files. He followed Lulu when she left work and kidnapped her. Lulu insisted that Dante would rescue her until Ronnie informed Lulu that Dante was locked up after evidence appeared that connected him to the attacks on the dancers.
Ronnie tied Lulu up in a hotel room but she managed to make enough noise to attract the attention of Sam McCall. Ronnie tried to pacify Sam's suspicions but he kidnapped her as well and took both women to the deserted Haunted Star casino. Dante and Lieutenant John McBain arrived shortly after and Dante confronted Ronnie over what he had done. Ronnie confessed that he had been attacking dancers for years after he discovered his wife had cheated on him. He blamed Lulu, Delores, and Dante for ruining his cover-up of the attacks by continuing their investigation into the crimes.
Dante tried to calm Ronnie down and offered to help his friend get professional help to deal with his actions. Ronnie told Dante it was too late to save him and he confessed that he had killed Officer Padilla's sister Rose and he had also been the one to shoot at Dante in the parking garage of the Metro Court after Dante had come close to discovering the truth about the attacks on the dancers. While Ronnie confessed his list of crimes, Sam managed to free Lulu and Ronnie fired a shot at her and grazed her arm.
Lulu and Dante escaped as Ronnie took Sam hostage when John entered. Ronnie held a gun to Sam and her unborn child and tried to use them as a shield against John. His plan worked until Sam pretended to faint which allowed John a clear shot at Ronnie. John took the shot and Ronnie died on the floor of the Haunted Star casino.