Born on February 15, 2045, Steph was a citizen of Canada and was raised Catholic by her mother, Joan. During the annexation of Canada by the United States of America in the 2070s, Steph and her mother were still living in Canada; Steph was issued a provisional U.S. citizenship card on September 1, 2071, listing her as a resident of the annexed territory of Canada.

Whether concurrently or later, Steph and her mother were interned at the Uranium City Internment Camp. They eventually escaped together with other imprisoned Canadians; however, they were cornered by a U.S. Marine in power armor. A Canadian rebel rushed the Marine with an explosive, killing the Marine and injuring Steph's mother, but leaving Steph unharmed. Joan urged Steph to leave for the border and survive through any means necessary, including killing Americans, whom Joan said should be dehumanized. Steph crossed the border, killing a civilian man for his food, as well as killing a border agent in order to cross a border checkpoint into the U.S. She eventually made her way to Las Vegas, Nevada, where she became a maid for the Lucky 38.

One night while working at the Lucky 38 in 2077, Steph was called upon to escort a drunken Hank MacLean out of Cooper Howard's hotel room. Knowing that Cooper was the face of Vault-Tec at the time, Steph asked him how to acquire a spot in the Vaults; Cooper told her to ask Hank. That night, Steph ingratiated herself to and seduced Hank MacLean until he proposed to her at 4 a.m. The two were eventually married. Hank got Steph into Bud Askins' executive assistant training-slash-Vault experiment program Bud's Buds; he may have done so without Bud's knowledge, as Bud was seemingly confused about Steph's presence in Vault 31 over the years. Steph was aware of her husband's secret ties to and work for the Enclave, though it is unclear to what degree she herself was involved. During her time at Vault-Tec, Steph's demeanor would often frighten her coworkers, including Betty Pearson.

As a Vault-Tec employee and member of Bud's Buds, Steph was placed into cryostasis in Vault 31; the program involved the Vault 31 dwellers being periodically thawed out and transferred to the connected Vaults 32 or 33 to manage their dwellers and participate in controlled breeding (eugenics) under Askins' supervision.

Reactivated sometime in or before 2294, Stephanie was transferred to Vault 33. By the time that Steph was awakened, Hank had been awake for decades; he had remarried, and had a family with, a woman named Rose despite him and Steph still being married. Steph eventually married Vault 33 dweller Bert on March 17, 2294. She is fond of Bert, though her memory of their wedding night is spoiled by Bert's lack of stamina in bed. Regardless, she developed feelings for him (perhaps spurred by his love of shoe maintenance), and by 2296, they were expecting their first child. Steph kept active during her pregnancy, participating in the Young Pipefitter's Association and keeping on top of her regular Vault duties. By 2296, Stephanie had also become best friends with Hank's daughter Lucy -- who was technically her stepdaughter -- even though Steph was closer in age to Lucy than to her first husband by this point. The two hoped to raise their children together once Lucy was married to a dweller from Vault 32 during the Triennial Trade in 2296.

Steph helps Lucy MacLean get ready for her wedding and shows Lucy where everyone writes their names and the date of their marriage. She and Lucy discuss the upcoming marriage, and Steph expresses excitement about raising their future children. Steph and other vault dwellers accompany Lucy and Hank to the connecting entrance between Vault 33 and 32, where Lucy and Monty first meet.

During the raid on Vault 33, Steph's husband Bert is killed in the cornfield. Steph cradles Bert's corpse before flying into a rage at the nearest raider. She is stabbed in the right eye with a fork during that fight, but she then uses a submachine gun to kill the raider who stabbed her before firing at several others. However, she is later captured by Lee Moldaver's men, along with Chet and several other vault dwellers, and are held hostage when Hank and then Lucy confront them at the entrance to Vault 32. She escapes from the situation along with everyone else after Moldaver leaves with Hank MacLean and detonates a bomb at the connecting entrance between Vault 33 and 32.