She is portrayed by Alison Tolman.
Mary Pat was happily married to Jeff. When Jeff suffered a heart attack in their home, Mary Pat was on the phone with 911 when she suddenly decided not to report her husband's death so that they would continue receiving his disability checks. She then chopped him up so his body would fit inside his meat freezer. ("Jeff")
Around six months after the death of her husband, Mary Pat was still struggling to juggle being the mother of their four young children, one of whom is a newborn, and making ends meet. Her friend Elena whom she possibly carpooled with invited her to join the Secret Shoppers, a seemingly harmless business formed by Beth Boland, Ruby Hill, and Annie Marks, for extra income. However, due to her busy schedule and looking after her children, she failed to successfully do the tasks assigned to her, thus unknowingly returning the fake money that should have been traded with real cash. She was subsequently fired by Beth, Ruby, and Annie, during which she realized that the business was associated with organized crime. She began to blackmail the ladies to force them to pay her $10,000 a month in exchange for her silence. ("Special Sauce")
Soon, she began tracking the women down to places they went to and worked to extort them for more money. When doing so with Annie at the grocery store at one point, Annie's manager Boomer saw her and tracked her down to her local church community. Boomer got close to her to plant a recording device in her home, hoping to get information from her to expose Annie's crimes. ("Summer of the Shark") The two soon began dating until the recorder ran out of batteries, leading Mary Pat to discover it. She approached Beth about it and told her about Boomer, and Beth told Mary Pat everything about him, including the fact that Boomer attempted to rape Annie. Mary Pat confronted Boomer, recording his statements about his encounter with Annie, and demanded that he leave. ("Remix")
Boomer kept coming back and Mary Pat kept turning him away, until one night he came with news that he was now a key witness in the case against the women. Boomer threatened to tell his FBI contact Jim Turner about Mary Pat's affiliation with the three women and convinced her that the only way he couldn't legally testify against her is if they were married. ("I'd Rather Be Crafting")
When Boomer went into hiding in a form of protective custody after the three threatened his life, Mary Pat helped them track down his motel by calling him and telling him she missed him, encouraging him to give up his location. While the girls went to him with plans to kill him, Mary Pat packed their things to escape. Boomer arrived before she could finish, however, and got angry when Mary Pat tried to break up with him and enumerated the reasons why. Boomer then raped her. While Boomer slept, Mary Pat decided to leave with her children but he tried to stop her minivan and got hit in the process. Mary Pat moved the body, ("You Have Reached The Voicemail Of Leslie Peterson") but Boomer had woken up and was gone by the time she returned. She then called Annie, Beth, and Ruby for help, playing it off as if Boomer died and thus convincing them to help get dispose of the body which was in fact her late husband Jeff's. ("Jeff")
Described as a "wolf in sheep's clothing," Mary Pat seems like a down-to-earth mother and breadwinner of her four young children. The lengths she would go to provide for her family will later reveal her as a blackmailer capable of fraud and other crimes.