Acker made her major television debut when she starred as
Winifred "Fred" Burkle in
Angel (seasons 2–5), and also as the character of
Illyria for part of the show's fifth and final season. She won the 2003
Saturn Award for Best Supporting Actress on Television for her portrayal. Acker joined the cast of
Alias in 2005 for its final season, playing villainess Kelly Peyton. Acker graduated from guest star to cast member as the show entered the final episodes in April and May 2006. Also in 2005, Acker provided voice acting for the character of
Huntress (Helena Rosa Bertinelli) on the animated series
Justice League Unlimited. in which she was reunited with
Angel co-star
Alexis Denisof (the real-life husband of
HIMYM star
Alyson Hannigan), who had a recurring role on the show. Acker portrayed Dr. Claire Saunders/Whiskey, a recurring character, on
Joss Whedon's
Dollhouse. She guest-starred in 10 of the 13 episodes of the first season and three episodes of the second. In 2010, Acker was a series regular in the
ABC drama
Happy Town, portraying the character Rachel Conroy. That same year, she starred in the season-one finale of the
Fox series
Human Target as the mysterious Katherine Walters. On May 25, 2010, she appeared on CBS's
The Good Wife. She appeared in the horror movie
The Cabin in the Woods, released April 13, 2012. In 2012, she made guest performances in
Warehouse 13,
Once Upon a Time, and
Grimm. Also in 2012, she starred as
Beatrice in Joss Whedon's film
Much Ado About Nothing. Acker appeared in season three of the sitcom
Husbands as Claudia, Brad Kelly's ex-fiancée who unexpectedly shows up. In March 2014, Acker was cast as
Audrey Nathan, a former lover to
Phil Coulson in an episode of
Agents of S.H.I.E.L.D. Between 2012 and 2016, Acker portrayed Samantha "Root" Groves on the
CBS drama
Person of Interest; Groves became a regular character starting in the series'
third season. Starting with the series' 100th episode, Acker took on the voice role of the Machine. In the series finale, she also appeared as the
visual manifestation of the Machine. In March 2017, Acker was cast as Caitlin Strucker in the pilot for the prospective Fox television series
The Gifted, which was picked up to series in May 2017.
The Gifted began airing on October 2, 2017, and Fox canceled the series after two seasons on April 17, 2019. Following the conclusion of
The Gifted, Acker was cast in 2019 as Kathleen Shepherd, the fourth sister of
Derek Shepherd, in an episode of ABC's ''
Grey's Anatomy''. She followed this with a recurring arc as Catherine in the third season of Fox's
9-1-1: Lone Star in 2022. Acker returned to a main cast role in 2023, starring as Tory in
Freeform's mystery thriller
The Watchful Eye. She also appeared in the 2024 theatrical film
Ordinary Angels, starring
Hilary Swank. ==Personal life==