Margaux Dawson made her first appearance on May 29, 2018, when she prosecuted Carly Corinthos for the attempted murder of Nelle Benson. Carly, who had insisted that Nelle's fall down the stairs during Nelle's baby shower had been an accident, was found not guilty by reason of insanity and sent to Ferncliff. Eventually, it was revealed that Nelle had set Carly up, and Carly's sentence was commuted. Margaux regretted the horrors that Carly had been subjected to at Ferncliff, but she vowed to take down Carly's husband, Sonny.

On July 17, 2018, Margaux's personal life took a turn when she bumped into Drew Cain at a hot dog stand in the park. Margaux accidentally squirted mustard on his shirt, and the two ended up bonding over baseball. A week later, Margaux found a flash drive in Metro Court Restaurant's lost and found. She quickly discovered that it contained Drew's memory mapping, but rather than turn over the evidence, which had been part of the Jason Morgan conspiracy investigation, she held onto it and kept quiet. Margaux hoped that Drew could be persuaded to help her take down Sonny by using Jason's memories, but Drew made it clear that he would not betray his brother.

On August 21, 2018, Margaux made a bold move by kissing Drew. She immediately regretted the kiss when Drew didn't return it. Humiliated, she fled Charlie's Pub. Drew gave chase and quickly caught up to Margaux. She admitted that she was attracted to him, and she'd thought the feelings had been reciprocated. Drew conceded that he was attracted to Margaux and wanted to get to know her, but he refused to help her take down Sonny. Margaux agreed to Drew's terms. After Margaux left, Drew called Curtis Ashford and asked him to run a background check on Margaux.

The following day, Curtis met with Drew to report what he had uncovered. According to Curtis, Margaux had grown up in Princeton, New Jersey, as an only child in a middle-class family. She'd had a full ride from NYU and graduated cum laude. Margaux had married a college sweetheart named David Dawson, but the marriage had ended amicably less than two years later. After the divorce, Margaux had attended Stanford for law school, and she'd graduated with honors in the top ten percent of her class.

Curtis went on to reveal that Margaux had worked as an assistant district attorney in Oakland, playing a significant role in "several high-profile cases" before moving on to Brooklyn then Port Charles, where she took a job as district attorney. Drew suspected that Margaux was hiding something that wouldn't show up on a background check, so he decided that, rather than vetting her in order to date her, he would date her in order to vet her.

On September 11, 2018, Margaux was sitting at the bar in Metro Court when Drew spotted her. Margaux confessed that she had been celebrating her father's birthday. She explained that she'd never known her father because he had vanished without a trace when she'd been two years old. Margaux conceded that her father had likely died, but she would not rest until she had answers. At the same time, across town, human remains were unearthed when a gas line in Charlie's Pub exploded. They were later revealed to be the skeletal remains of Margaux's father, Vincent Marino.

The discovery of Vincent Marino's remains renewed Margaux's efforts to prove that her father had been murdered. She was certain that Sonny had been the killer or knew who the killer was, since her father had been a consigliere for Joe Scully's criminal organization. Margaux used every tool in her arsenal to force Sonny to confess, including questioning his father, who suffered from Alzheimer's disease, and having her father's remains interred near Morgan Corinthos' grave. Sonny refused to break.

Margaux once again appealed to Drew for help, but he refused. However, Sonny saw a picture of Margaux's mother, and the pieces fell into place for him. With Sam and Spinelli's help, Sonny gathered the proof that he needed then contacted Margaux.

On November 9, 2018, Margaux and Sonny confronted Jeannette Marino about her affair with Joe Scully. Initially, Jeanette denied that she'd had an affair with Joe, but Margaux showed her mother the love letters that Sam and Spinelli had found. Jeanette defended herself by claiming that she'd been in a loveless marriage, but Margaux had heard enough. She disowned her mother then ordered her mother to move out of their family home. Margaux opted not to pursue legal charges against her mother for Jeanette's role in Vincent's murder, which meant that Sonny was spared, as well.

Margaux tried to pick up the pieces by making things right with Drew. She gave him the flash drive with his memory mapping then focused on her work. Margaux and Sonny became friendly, which caused tension in Sonny's marriage for a short time.

As 2018 drew to a close, Margaux became interested in the teachings of a man named Shiloh who was head of the Dawn of Day organization. DOD helped impoverished communities through outreach programs. Shiloh had also written an inspirational book about his journey of enlightenment, and he offered seminars at the local DOD house -- for a price.

Margaux felt a connection with Shiloh, and she was slowly drawn into the heart of the organization. In May 2019, several citizens began to raise questions about DOD and its charismatic leader. Margaux defended Shiloh, certain that all the good work that he did for various communities was a testament to who Shiloh was. However, all that changed on June 5, when Sonny invited Margaux to his home.

Jason and Sonny confronted Margaux with irrefutable proof that Shiloh had drugged Sam Morgan, tattooed her, and then attempt to sexually assault her as part of an initiation ceremony into the inner sanctum of DOD known as the Trust. Margaux was shocked when Sonny explained that Jason had secured the pledges that members had given to Shiloh as a show of good faith to join the Trust -- including Margaux's. The pledges were dark secrets that had the capacity to destroy the pledge's life if it were to become public. Sonny vowed to expose Margaux's pledge if she did not arrest Shiloh.

Margaux pointed out that Sonny was no better than Shiloh, but she had an arrest warrant issued for Shiloh then instructed Detective Harrison Chase to arrest the cult leader.

To Margaux's surprise, Sonny handed her the pledge unopened. Margaux paid Shiloh a visit at the hospital just as Chase had slapped on the handcuffs. Shiloh was recovering from his encounter with Jason when Jason had stopped Shiloh from raping Sam. Shiloh appealed to his disciple to right the "injustice" done to him, but Margaux informed him that she had heard the recordings of his attempt to rape Sam. Annoyed, Shiloh switched tactics by threatening to expose her role in covering up her father's murder, but Margaux pulled out the pledge that Sonny had returned to her.

Margaux explained that all the pledges had been destroyed, and she had the only copy of hers that was left. Undaunted, Shiloh warned her that the tale alone would be enough to destroy her career, but Margaux smiled. She revealed that she had decided to resign. Stunned, Shiloh watched as Margaux walked out. A few minutes later, Margaux told Mayor Laura Webber Collins of her intention to resign.