Miller was born and raised in Wyckoff, New Jersey. Miller has two older sisters, Saiya and Caitlin. Miller’s mother, Marta Miller (née Koch), is a modern dancer. Miller’s father, Robert S. Miller, was senior vice president and managing director of Hyperion Books, and later became a publisher at Workman Publishing. Miller’s father is Jewish and Miller’s mother, who has German and Dutch ancestry, is from a Christian background. Miller self-identifies as Jewish and “spiritual.” At the age of six, Miller started to train as an opera singer, in order to overcome a speech impediment. Miller has sung with the Metropolitan Opera, and performed in the American premiere of Philip Glass’s opera White Raven. Miller attended Rockland Country Day School and The Hudson School, dropping out at age 16 after the release of the film Afterschool.
Miller’s start in film began in 2008 with Antonio Campos’ Afterschool, in the role of a teenager at a boarding school who accidentally films the drug-related deaths of two classmates, and is then asked to put together a memorial video. The following year, Miller appeared in City Island with Andy García, Julianna Margulies, and Steven Strait. In 2010, Miller portrayed the lead of Beware the Gonzo and had a supporting role in Every Day, both of which premiered at the Tribeca Film Festival. Miller next appeared in the BBC Films film We Need to Talk About Kevin (2011), alongside Tilda Swinton and John C. Reilly, which was adapted and directed by Lynne Ramsay from American author Lionel Shriver’s 2003 novel of the same name. Miller played Damien on the Showtime hit comedy series Californication. Miller appeared on Royal Pains as Tucker Bryant for two seasons. In the 2012 film adaptation of the novel The Perks of Being a Wallflower, Miller played Patrick, alongside Logan Lerman and Emma Watson. Miller has been credited with singing, drumming and percussion on musical recordings by the band Sons of an Illustrious Father, as early as 2011. The band is a trio that features Lilah Larson (vocals, guitar, bass and drums), Josh Aubin (bass, keyboards, guitar, vocals) and Miller. In 2019, the band released a cover of “Don’t Cha” by the Pussycat Dolls and Miller appeared in its music video. Miller played Credence Barebone in the 2016 film Fantastic Beasts and Where to Find Them, a spin-off of the Harry Potter film series. Miller reprised the role in the film’s sequel, Fantastic Beasts: The Crimes of Grindelwald, which was released in November 2018. Miller is set to reprise the role in the Untitled third Fantastic Beasts film, to be released in July 2022. Miller portrays Barry Allen/The Flash in Warner Bros.’ DC Comics adaptations, first appearing in cameos in Batman v Superman: Dawn of Justice and Suicide Squad, and continuing the role as one of the leads in Justice League. Miller is set to play the character in the standalone film The Flash, which is set to be released in 2022. Miller attended Middle East Film and Comic Con in 2018 representing their character in the DC Comics franchise film Justice League. In 2020, Miller reprised the role of The Flash for a cameo appearance in the Arrowverse crossover event, Crisis on Infinite Earths. In 2020, it was confirmed that Miller would portray Trashcan Man in the upcoming CBS AllAccess television miniseries The Stand.