Stephanie Kay Johnson was the daughter of Steve and Kayla Johnson. In 1991 Stephanie was kidnapped by her nanny, Sheila Salsbury, but was later recovered in Australia. After Steve's death, Stephanie moved to Los Angeles with her mother, and wasn't seen again until she was a grown woman, at which point she was a racecar driver. After a brief relationship with Max Brady, Stephanie shortly left town, only to return after her father, recently returned from the dead, began experiencing problems. She was, by then, romantically involved with Jeremy Horton.

She talked Chelsea and Max into working with and investing in Jeremy's business, "Touch the Sky Airlines." But, the business turned out to be a counterfeit clothing smuggling operation, and Max lost all of his money. He did, however, manage to fall for Stephanie, and the two began a flirtation. But, Stephanie told Max that she was committed to Jeremy and couldn't be with Max. Max began dating Morgan Hollingsworth. Soon, Jeremy left town to avoid the police, and Stephanie was left to sort out her feelings for Max.

Stephanie was so jealous of Max and Morgan that she went out one night to a party and went home with Ford Decker. She thought that she would just get her mind off Max, but instead Ford drugged her drink and when she was subdued, he raped her. Stephanie didn't know how to deal with the rape; and, despite the fact that Ford continued to drug and rape other women on campus, Stephanie could not find it in herself to testify against him. Instead, Stephanie conspired with her sorority sisters to lure Ford to their house and drug him to teach him a lesson. But, when things went wrong and Ford tried to drug and rape Chelsea, he chased her up the stairs and then fell to his death. Stephanie convinced the other girls that they should not call the police, but instead, they should hide the body themselves. Stephanie called Max to help move the body. The sisters kept the secret for a few weeks until Chelsea finally told Ford's dad where Ford's body was. No charges were brought against Stephanie.

The rape brought Max and Stephanie closer. Stephanie and Max became inseparable. Stephanie even helped Max find his biological sister, Melanie, in France. But Stephanie and Melanie did not get along, and the rivalry began to take a toll on Max and Stephanie's relationship. Max told Stephanie that Melanie confessed to a crime. Stephanie turned Melanie into the police. Furious that Stephanie would break his trust, Max broke up with Stephanie.

Stephanie set out to pick up the pieces after her break up with Max. She took an internship at Titan and began working with Philip Kiriakis. Sparks flew between Philip and Stephanie. Stephanie supported Philip through his mother's cancer and soon the two began dating. Unfortunately, Melanie would be a problem with this relationship too. Melanie became a consultant for Titan and set her sights on Philip as well.

But, Stephanie was the only woman that Philip wanted. He proposed to Stephanie and she accepted. But, before they could enjoy their engagement, Stephanie became the latest target in the DiMera/Kiriakis feud. EJ ordered Owen Kent to kidnap Stephanie, which he did. While he was waiting to turn Steph over to EJ, Owen became obsessed with Stephanie. He wanted to run away with her. Steph allowed Owen to kiss her in hopes that she could distract him enough to escape. But her plan didn't work. Luckily, Philip, Melanie, and Brady found where Stephanie was being kept and were able to rescue her.

Stephanie started suffering from nightmares and daydreams about her time being held captive. She went to see a doctor who prescribed her some pills to help her sleep and deal with the anxiety. She never fully recovered from the trauma and blamed her danger on Victor and Stefano's feud. She insisted that Philip break off all ties with his father. When he refused, she broke up with him. Philip tried to get her back and the two reconciled for a brief time. But, Stephanie broke up with Philip for good when she learned that he had slept with Melanie during one of the periods when he and Stephanie were on a break.

Stephanie set her sights on Nathan Horton, a young doctor who had been dating Melanie. They went on a few dates, but Stephanie saw that Nathan still had feelings for Melanie. As Melanie and Philip grew closer, Stephanie did everything she could to keep Nathan away from Melanie, including tearing up a love letter that Melanie asked Stephanie to give to Nathan. Melanie and Philip eventually married, and Nathan moved in with Stephanie. But Stephanie still worried that if Melanie was ever single again, Nathan would run back to her.

When Stephanie learned that Philip had cheated on Melanie with Chloe, Stephanie convinced her friend, Ian, to hack into the hospital's data base to see the results of Chloe's paternity test. Stephanie learned that Philip was the father of Chloe's baby. But, someone changed the results of the test before Chloe saw them. Stephanie knew that Philip was the real father, but kept quiet so that Melanie wouldn't leave Philip. When the truth came out and Nathan found out that Stephanie had kept the secret, he broke up with her.

While Stephanie has not been seen on screen since April 2011, she is supposedly still living in Salem. It was later revealed that Stephanie had taken a job and moved to Chicago.

Stephanie returned to Salem in February 2017 for her parent's wedding.