Irish actress Katie McGrath did not intend to make a career for herself in the acting profession. Upon graduating from Trinity College in Dublin she became interested in fashion journalism and worked for lifestyle and fashion magazine, Image. She left after a while, as it was not her calling. After this, her mother's best friend, an assistant director, helped get her a job as a wardrobe assistant on the series The Tudors (2007). Whilst working on the production some of the staff suggested she try acting. A cast driver on the series passed her a list with the actors' agents on it, so she wrote and sent photos to them, and was signed shortly after.

In television, she is best known for portraying Morgana on the BBC One series Merlin (2008-2012), Lucy Westenra on the British-American series Dracula (2013-2014), Sarah Bennett in the first season of the Canadian horror anthology series Slasher (2016) and for her role as Lena Luthor on the American superhero series Supergirl (2016). Her film roles include Lady Thelma Furness in the drama film W.E. (2011), Zara with one of the most iconic scenes in the whole franchise in the science fiction adventure film Jurassic World (2015) and Elsa in the epic fantasy film King Arthur: Legend of the Sword (2017).

McGrath was raised in Ashford, County Wicklow, Ireland, by Paul, who works with computers, and Mary, who works for an Irish designer. She has two older brothers, Sean, an online media manager, and Rory, who is a post-production producer. She studied the International Baccalaureate at St. Andrew's College before graduating from Trinity College, Dublin with a degree in history with a focus in Russian history.

McGrath was cast in Damage, an Irish TV-movie in 2007. She also starred in the play La Marea at the Dublin Theatre Festival in the same year. She appeared in the feature films Eden and Freakdog in 2008, before being cast as Morgana Pendragon in Merlin.

In 2009, McGrath starred in a five-part docudrama for Channel 4 exploring the life of Queen Elizabeth II, The Queen, in which she played a young Princess Margaret. Emilia Fox portrayed Elizabeth II in the same episode in which McGrath appeared; the two had previously worked together as sisters Morgause and Morgana in Merlin. In 2010, McGrath was cast in Madonna's directorial debut W.E., an Edward VIII biopic. McGrath played Lady Furness, the king's former mistress who introduces him to Wallis Simpson.

2011 saw McGrath film the comedy-drama A Princess for Christmas in Romania. In September 2011, McGrath lent her voice to the characters in the Irish animated short film Tríd an Stoirm (Through the Storm). Later that month, McGrath was cast as Oriane Congost in Labyrinth.

McGrath was reunited with her The Tudors co-star and friend Jonathan Rhys Meyers in NBC and Sky Living's horror drama TV series Dracula; she portrayed Lucy Westenra. In June 2013, McGrath co-starred in episode four of the Channel 4 show Dates as a young lesbian on the dating scene alongside Gemma Chan. In November 2014, McGrath co-starred in a Hozier music video for the song "From Eden".

In 2015, McGrath had a supporting role as Zara in the film Jurassic World and starred in the Crackle original spy-thriller, The Throwaways.

McGrath portrayed the lead role in the first season of Chiller's original horror series Slasher, which premiered on 4 March 2016.

In 2016, it was announced that McGrath would play the recurring role of Lena Luthor in the second season of Supergirl. She appeared in the season two premiere episode entitled "The Adventures of Supergirl" and was promoted to a series regular in March 2017 for the third season. She appeared as Elsa in Guy Ritchie's fantasy film King Arthur: Legend of the Sword, which was released in May 2017.

In 2016-2017 McGrath was featured in the first two seasons of Frontier, alongside Jason Momoa. In 2019 she starred in the Australian TV show Bridesmaids, alongside Georgina Haig and Abbie Cornish. In 2020 McGrath narrated the audio book for Islands of Mercy by Rose Tremain.

After wrapping up the sixth and final season of Supergirl in 2021, McGrath traveled to Budapest, where she shot the John Wick spin-off series The Continental. McGrath can be seen as The Adjudicator in The Continental, which aired its three episodes in 2023.