Alexander got her acting start on two short-lived series: the medical drama
Presidio Med and ABC's twenty-something drama
Wasteland. She achieved widespread media attention and critical praise when she appeared in the fourth season of ''
Dawson's Creek as Gretchen Witter, the sister of Pacey Witter, and dated the title character. Alexander was also in an episode of the short-lived Fox comedy series Greg the Bunny, in which she played the role of a lesbian TV Guide'' reporter and shared an onscreen kiss with
Sarah Silverman. Alexander appeared in the films
Lucky 13 and
All Over the Guy, as well as ''
He's Just Not That Into You (in the role of Catherine), the independent movie The Last Lullaby, and Yes Man (as Lucy) starring Jim Carrey. She appeared as District Attorney Robin Childs in the CSI
episode "Alter Boys", and she appeared in an episode of Friends'' ("
The One with Joey's Interview"), playing a
Soap Opera Digest reporter who interviewed
Joey. Alexander joined
NCIS as
Secret Service/NCIS Special Agent
Caitlin "Kate" Todd in 2003. Her character replaced Vivian Blackadder, played by
Robyn Lively, who had appeared in the
backdoor pilot episodes "
Ice Queen" and "Meltdown" on
JAG. The official pilot episode "
Yankee White" aired September 23, 2003, on
CBS. Alexander's two-year tenure on the show ended in May 2005 when her character, Kate, was killed by a terrorist who shoots her in the head during the last few seconds of the season-two finale, "
Twilight". The official explanation for her departure was that "she asked to be let out of her contract to pursue other opportunities". In statements made by Alexander since her departure from
NCIS, she has made clear that her problem with the show was that she felt the workload was too physically demanding for her. In subsequent television series work, episode counts were considerably smaller, and her character portrayals required less physical effort than her role on
NCIS. In her final appearances in the two-part episode "
Kill Ari", Alexander was credited as a special guest star. Her character Caitlin Todd reappeared in a "what if" scenario with
Anthony DiNozzo for the
NCIS 200th episode "
Life Before His Eyes" in 2012, however, Alexander did not return to film new footage for the episode; rather footage from the second season episode "
SWAK" was recut to incorporate her into the new episode. She also had a small role in 2006's
Mission: Impossible III. She joined the cast of
The Nine in the role of Nick's ex-wife. From 2010 to 2016, Alexander played Dr. Maura Isles for seven seasons on the
TNT series
Rizzoli & Isles – the Commonwealth of Massachusetts' Chief Medical Examiner, who works with the Boston Police Department. Alexander co-starred with
Angie Harmon, who played Boston Homicide Detective
Jane Rizzoli, Isles's best friend on the show. In 2015, Alexander began a recurring role on Showtime's
Shameless portraying Helene Runyon, one of Lip Gallagher's college professors. Her character Helene maintains an open relationship with her husband Theo (played by
Michael Reilly Burke) which allows her to have sex with Lip. The relationship between Helene and Lip is both romantic and one which provides him with guidance in his college career, but the affair blew up in season six (2016). Alexander was cast opposite
Ludacris in the 2018 drama film
The Ride. In 2018, Alexander guest-starred in the
Law & Order: Special Victims Unit episode, "Caretaker" (season 20, episode 7), as villainess Anna Mill. Alexander played Detective Chesler in the 2020 Netflix film,
Dangerous Lies. In 2020, Alexander voiced
Adeline Kane in the animated web-series
Deathstroke: Knights & Dragons, based on
DC Comics. In 2021, Alexander directed the 9th episode "Red Flag" of season 3 of the Netflix show
You. == Personal life ==