Mary Pat Ingles was the head nurse at Ferncliff when Carly Corinthos was admitted. Carly had been found not guilty by reason of insanity for the attempted murder of Nelle Hayes and her unborn child, Carly's grandson. Mary Pat resented rich and privileged people like Carly, and she didn't hide her dislike for Carly in particular. Mary Pat took pleasure in tormenting "Caroline," and she gave Carly a set of strict rules to follow. Dr. Kevin Collins was the only person who stood between Mary Pat and Carly. When he discovered that Mary Pat had a fondness for controlling patients by giving them powerful sedatives, he made it clear that Carly was not to be given any medications without his approval, but Mary Pat had little regard for the doctor's orders.

On June 25, 2018, Mary Pat discovered that her newly hired janitor was really Jason Morgan, who had taken the job to protect Carly. Mary Pat was furious and fired Jason. Later, it was revealed that Kevin had told Mary Pat about Jason because Kevin worried that if Mary Pat discovered that he'd known about the ruse, he might have been fired -- leaving Carly unprotected. The following day, Carly asked about the patient in the room next to hers, who had been communicating with her through Morse Code. Mary Pat warned Carly to stay away from the patient, whom Mary Pat described as dangerous. Later, Mary Pat entered the man's padded cell, reminded him that she had warned him what would happen if he gave her any trouble, then injected him with a potent sedative.

On July 13, 2018, Mary Pat was forced to call Kevin to let him know that Carly was agitated and had thrown a chair at a door. Mary Pat warned him that she would restrain and sedate Carly, but Kevin instructed Mary Pat to not do anything because he was on his way. Carly was upset because her son was about to marry Nelle Hayes. Nelle had been gaslighting everyone in Port Charles in a twisted effort to destroy Carly's life. Carly was desperate to stop the wedding, but Kevin explained that she couldn't leave Ferncliff.

Kevin decided to instruct Mary Pat to give Carly something for Carly's panic attack. Delighted, Mary Pat taunted Carly as she approached the bed. However, Carly quickly turned the tables and managed to inject Mary Pat with the syringe. After Mary Pat passed out, Carly escaped the asylum. However, Carly was quickly captured and returned to Ferncliff.

Mary Pat was furious. Carly was confined to a straitjacket and placed in a padded cell. Mary Pat entered carrying a syringe then gleefully informed Carly that Dr. Lasiris had taken over Carly's treatment. Carly desperately tried to refuse treatment, but Mary Pat made it clear that Carly did not have any say in the matter then injected Carly with a powerful sedative. Mary Pat also revealed that she had confiscated Carly's hidden cell phone.

On July 26, Mary Pat took a heavily sedated Carly for an electroconvulsive therapy treatment with Dr. Lasiris. Carly saw another patient exiting the room who looked exactly like Kevin. The man whispered something to Carly as they passed each other, prompting Carly to realize that he was the same patient who had been communicating with her through Morse Code. However, her muddled thoughts soon turned to terror when she realized that she was being strapped to a table in preparation for electroconvulsive therapy. Carly was saved at the last possible moment when Jason, with some help from Franco, rescued Carly from Ferncliff.

On August 3, Sam and Curtis paid Ferncliff a visit, posing as officers from "United Mental Health Institute of America." They claimed that Ferncliff was on a short list for a special award. Mary Pat was eager to help them, unaware that they were really looking for evidence that Carly had been given treatment without proper consent. Sam and Curtis found what they were looking for when Mary Pat gave them access to patient files.

On October 26, Carly returned to Ferncliff with Laura Collins. They were determined to make certain that changes had been made to the facility since Carly's horrific ordeal, and to check on the patient who had haunted Carly's dreams. Carly and Laura bumped into Mary Pat, who had collected the last of her personal belongings after being fired.

On October 30, Mary Pat was at the Floating Rib when "Kevin" approached her at the bar. She had no idea that the man she was talking to was his homicidal twin, Ryan Chamberlain, but she wasn't surprised to see "Kevin." Mary Pat had suspected that the doctor would seek her out because she knew his secret. "Kevin" lured her to the piers under the pretext of talking privately.

On the pier, Mary Pat wondered how the world would react if they learned that "Kevin" had been hiding a homicidal maniac at Ferncliff. "Kevin" reminded Mary Pat that his twin had been her patient, too, but Mary Pat didn't care about Ryan Chamberlain because she needed money. Mary Pat threatened to sell her story to the Invader, so "Kevin" offered to pay her until she found another job. Mary Pat wanted more; she demanded an easy desk job at the hospital and a steady income from "Kevin."

After Mary Pat walked away, confident that she had exactly what she wanted, Ryan grabbed a longshoreman's hook that he had found on the pier then followed her. Hours later, Mary Pat's decapitated head was found floating in a barrel during a Halloween charity event at the Floating Rib. Mary Pat's body had been found displayed with a pumpkin perched on its shoulders at the Jerome Gallery.

On November 5, 2018, Jordan Ashford received the results of the autopsy report. According to the report, Mary Pat's death had been instantaneous, and she had been struck from behind with a "knife or a hook," which had pierced her heart. The wound had been precise, so that the bleeding had been internal, suggesting that the killer had been a surgeon or someone with medical knowledge.