[3] Moriarty was cast in her first professional roles as a teenager, acting in the television series One Life to Live and Law & Order: Special Victims Unit.

Her first roles of note were as Vince Vaughn's daughter in the 2012 comedy film The Watch;[6] Kelly, the love interest of Nick Robinson's character, in the 2013 indie comedy The Kings of Summer;[7][8] and Natalie, "a daughter destined to find out who killed her father, who once had ties to organized crime", in the ABC drama series Red Widow.

[14] In the show's first season, which was released in November 2015, Moriarty played Hope, a college student whose life is ruined by Kilgrave, the main villain.

[16] Her role as Gibson's daughter in Blood Father received praise from critics such as Manohla Dargis of The New York Times,[17] Alonso Duralde of TheWrap,[18] and Owen Gleiberman of Variety,[19] but Ignatiy Vishnevetsky of The A.V.

Moriarty also had a supporting role in the critically acclaimed film Captain Fantastic, in which she plays the love interest of George MacKay's character.

[23] In December 2017, she was cast as Annie January / Starlight in The Boys, Amazon Studios's loose adaptation of Garth Ennis and Darick Robertson's comic book of the same name.

[26][27] In March 2024, Moriarty was cast in the neo-noir thriller Lips Like Sugar, alongside Woody Harrelson, Owen Wilson and others.