As an actress, Johnson is best known for her roles as Kimberly Hart on Mighty Morphin Power Rangers (1993–1995), Julie Emrick on Felicity (1998–2000), and Jules Callaghan on Flashpoint (2008–2012).

[3] Though born in Hyannis, Johnson grew up in Dennis, Massachusetts, and attended Dennis-Yarmouth Regional High School.

Despite the series being a huge success and having brought Johnson international recognition as an actress, the show brought her little financial security, as she and the others were paid only $600 a week for their work on the show, which included stunt work, and public appearances; none of the cast received any royalty payments from re-runs of episodes they appeared in.

[5] During Johnson's time with the franchise, whose productions were non-union and not subject to safety codes standardized in union contracts, Johnson faced multiple instances of danger of physical harm; while filming the television series, she was almost set on fire during a stunt, and during Turbo: A Power Rangers Movie, she was almost electrocuted.

[6][7] Johnson ultimately made the decision to leave the show in 1995, passing the role of the Pink Ranger to Australian actress Catherine Sutherland.

In January 2023, Johnson announced that she had declined to reprise the role of Kimberly Hart in the 30th anniversary Power Rangers special, Mighty Morphin Power Rangers: Once & Always, on Netflix but that she wished the special and her former co-stars well while also saying that she had other fun things in store for her fans that year.

[14] After she left the series in November 1995, Johnson went on to star in Disney Channel's Susie Q and in the Saved by the Bell: The New Class episode "Backstage Pass."

In 1997, she starred in NBC's adaptation of Lois Duncan's novel Killing Mr. Griffin and played a gymnast with an eating disorder in Perfect Body.

In the early 2000s, Johnson had roles in Interstate 60, Pursuit of Happiness, and Infested, as well as television film Hard Ground.

In the latter half of the decade, she had recurring roles in Wildfire and What About Brian, and she starred in television films Magma: Volcanic Disaster on Syfy and Fatal Trust on Lifetime.

Beginning in 2008, Johnson became a series regular on Flashpoint as Constable Jules Callaghan, a member of the fictional Strategic Response Unit of the Toronto Police service.

[21] She is a member of Film Fatales, a non-profit organization that advocates for women's parity in the entertainment industry.

In December 2007, she contributed guest vocals to Koishii & Hush's cover of The Cars track "Since You're Gone", which was released as a single.