Horgan has appeared in films, such as Valiant (2005), Imagine Me & You (2005), Death of a Superhero (2011), Man Up (2015), Game Night (2018), Military Wives (2019), Dating Amber (2020), Together (2021), Everybody's Talking About Jamie (2021), and The Unbearable Weight of Massive Talent (2022).
Her second-youngest brother, Shane, is a sports pundit and retired professional rugby player, and the other children in the family work in the arts.
At the age of 27, Horgan started a degree in English and American Studies at Brunel University in west London, graduating in 2000.
Her first credited appearances on television were in The State We're In (2002) and Monkey Dust (2003), two sketch shows based on news and current affairs.
Her first named acting role on television was as Theresa O'Leary in Absolute Power (2003), a comedy set in the world of public relations and starring Stephen Fry.
In 2005 she made her big-screen debut as Beth in Imagine Me & You, a British-American romantic comedy with Lena Headey directed by Ol Parker.
The US/UK comedy series was written by David Cross, who also appeared as the title anti-hero, an incompetent American who takes a job leading the London sales team for an energy drink.
In September 2011 she appeared in the world premiere of Saul Rubinek's play Terrible Advice at the Menier Chocolate Factory in London.
In the sketch show, famous women from history are psychoanalysed by Rebecca Front's therapist; she played the novelist Jane Austen in the pilot, and later characters included Eva Peron, Cleopatra, Boudicca and Carmen Miranda.
Horgan also appeared in 2020’s second series of Criminal UK, BBC’s The Borrowers (2011), Dead Boss (2012) and Channel 4’s Free Agents (2009) and Bad Sugar (2012).
Horgan also appeared in the feature film adaptation of the West End musical Everybody’s Talking About Jamie as Miss Hedge, which was released in September 2021.
She played Donna, an irresponsible marketing manager who calls off her wedding at the last minute, and one of three women sharing a flat in Penge, south London.
In April 2015, HBO announced it had picked up the series after the pilot episode, and the show is Parker's first major acting commitment since Sex and the City.
Horgan co-created and executive produced Shining Vale, alongside Jeff Astrof: a horror-comedy series for Starz.
She later directed an episode of Amazon’s anthology series Modern Love titled “Rallying to Keep the Game Alive” which starred Tina Fey and John Slattery.
[26] In January 2013 in Secrets of a Good Marriage she discovered how various couples make their relationships work;[27] and in On the Verge of a Midlife Crisis, she spoke to six women who had coped with the experience.
[5] In 2014, Horgan established Merman, an independent production company, with Clelia Mountford, who produced A Young Doctor's Notebook, Mr. Sloane, and Cockroaches.
Áine, played by writer Aisling Bea, is a whip smart English-as-a-foreign-language teacher trying to pull her life back together after a "teeny little nervous breakdown".
Merman produces Channel 4 and Amazon's Frank of Ireland (2021–present), created by Domhnall and Brian Gleeson, the hilarious story of a man's hapless search for respect.
Merman has co-produced its first feature, Herself (2021) alongside Element Pictures, a female-fronted film that follows a broken family's journey to find a home set during the Irish housing crisis.
Herself had its debut at Sundance in January 2020, was selected for the London Film Festival in October 2020, and was released in Irish and UK cinemas on 10 September 2021.
Merman co-produces 2021's animated comedy Housebroken for Fox co-created with Clea Duvall, Jennifer Crittenden, and Gabrielle Allan.
Merman has recently announced their Starz horror-comedy series Shining Vale, starring Courteney Cox, Greg Kinnear, and Mira Sorvino.