She also portrayed Mia Smoak in the superhero series Arrow and starred as Julie Lawry in the post-apocalyptic miniseries The Stand.

[7] While growing up, McNamara was an advanced student in certain subjects, resulting in her being partly homeschooled; she received her high school diploma at age 14.

[8] At age 17, McNamara graduated summa cum laude with a Bachelor of Science degree in Business Administration from Drexel University, completing much of her course work online.

[5][15][16] She began making guest appearances in television series, including Law & Order: SVU, 30 Rock, and Drop Dead Diva.

[23] On May 6, 2015, ABC Family (now Freeform) announced that McNamara would be playing the lead character Clary Fray in Shadowhunters, a television adaptation of Cassandra Clare's bestselling book series The Mortal Instruments.

[31] In August 2021, The CW announced that McNamara would return as Mia Queen in the five-episode "Armageddon" storyline at the beginning of The Flash Season 8.

[32][33] In October 2019, McNamara was cast as Julie Lawry in the CBS All Access limited series adaptation of Stephen King's The Stand.

[34] Principal photography on the limited series was completed in March 2020, and CBS All Access announced a premiere date of December 17, 2020.

[35][36][37] In November 2018, McNamara joined the cast of a Charlie Day film, formerly known as El Tonto, and eventually retitled Fool's Paradise before a release in 2023.

[45] On April 11, 2022, she began hosting a Shadowhunters rewatch podcast with co-star Dominic Sherwood called Return to the Shadows.

[46] McNamara also starred in the Amazon Prime film Sugar (marketed on Lifetime as Danger Below Deck) alongside Jasmine Sky Sarin which was loosely based on the real-life story of the Cocaine Cowgirls.

[63][64] McNamara has also worked with the Lollipop Theater Network, visiting children who are confined to hospitals due to the severity of their illnesses.

[68] In 2018 and 2019, she participated in the Big Slick Celebrity Weekend, which raises donations for the Kansas City Children's Mercy Hospital.