Brown Findlay grew up in Cookham, Berkshire, United Kingdom, where her mother is a teaching assistant and her father is a financial adviser. She told Vanity Fair in 2012, “I grew up there, as did my Mum. My Nan and Granddad are around the corner. It is a very familiar place and incredibly dear to my heart. It’s sort of quiet, but wonderfully so.”

She trained with the National Youth Ballet and the Associates of the Royal Ballet, and at age 15 was invited to dance with the Kirov at the Royal Opera House for a summer season. She attended Furze Platt Senior School in Maidenhead. At the end of her GCSEs, she was accepted to a number of ballet schools, but chose to go to the Arts Educational School because of the A-level courses it provided and its pastoral care. Brown Findlay attended for two years: in her second year, she had three operations on her ankles, the last of which went wrong. After encouragement from an art teacher, she finished her education at Arts Educational School, Tring Park and then moved on to a Fine Art course at Central Saint Martin’s College of Art and Design. In a 2012 interview with Vanity Fair, Brown Findlay stated, “Growing up, I was completely in love and infatuated with ballet. Ballet was my life completely.” However, after three ankle operations, she was forced to leave dancing behind and ended up at university in London, where she discovered the stage. She says, “Acting was the element from ballet that I actually loved and missed the most.”

Almost immediately after shooting her first major role, in Albatross, Brown Findlay landed the role of Lady Sybil Crawley in the ITV period drama television series Downton Abbey, which became her breakthrough role, as the youngest of the Grantham daughters. In a 2012 interview with Vanity Fair, Brown Findlay stated, “I thought this character of Sybil was fascinating, and I liked her modern attitude to life.” Brown Findlay was the first major cast member to leave the series, when her character, the forward-thinking Lady Sybil, died from eclampsia after giving birth in the third series. During a 2015 interview, Downton Abbey creator, Julian Fellowes, discussed the plot twist, “Jessica [Brown Findlay, who played Sybil,] had said she was going to leave right from the beginning. She said, “I’m doing three years, then I’m leaving”, So that was all worked out,” Fellowes said of Lady Sybil’s death, which occurred shortly after she gave birth to her and Tom Branson’s baby daughter.

In 2012, Brown Findlay became the face of Dominic Jones’ jewellery line, and she was cast in Not Another Happy Ending by John McKay, and in the miniseries Labyrinth, based on the novel of the same name written by Kate Mosse, portraying Alaïs Pelletier. In 2012, Brown Findlay was cast as Beverly Penn in the film adaptation of the novel Winter’s Tale (2014) with Colin Farrell and Russell Crowe.

In March 2015, it was confirmed she had been cast in the remake of The Crow, in which Brown Findlay portrayed Shelly.

In July 2015, Brown Findlay played the role of the emotionally conflicted stepmother Alice Aldridge in The Outcast, the BBC’s two-part television adaptation of Sadie Jones’ novel.

In May 2015, Brown Findlay made her professional theatre debut at the Almeida Theatre, London, as Electra in a new adaptation of The Oresteia to positive reviews. The production later transferred to the Trafalgar Theatre in London’s West End. The writer and director was Robert Icke, who cast Brown Findlay in his production of Uncle Vanya at the same venue in February of the following year.

In September 2016, it was announced she would play Ophelia in a new production of Hamlet at the Almeida Theatre in London. The production was critically acclaimed and later transferred to the West End, where it ran until September 2017 with award-winning “Sherlock” actor Andrew Scott as Hamlet.

In 2016, she joined the cast of a biopic feature film initially entitled Steven, about the early life and career of English singer Morrissey, who co-founded the indie rock band The Smiths. The film, renamed England is Mine, premiered at the Edinburgh Film Festival in 2017, with Dunkirk actor Jack Lowden in the lead role.

Brown Findlay started dating actor Ziggy Heath in late 2016. They married on 12 September 2020.

In 2017, Brown Findlay portrayed Charlotte Wells, a brothel owner’s daughter and famed courtesan, in Harlots, a period drama television series created by Alison Newman and Moira Buffini. The series premiered on 27 March 2017 on ITV Encore in the UK and on 29 March 2017 on Hulu Plus in the US. The TV series focuses on Margaret Wells, who runs a brothel in 18th century England and struggles to raise her daughters in a chaotic household. The series is inspired by “The Covent Garden Ladies” by Hallie Rubenhold. Also in 2017, she voiced the character of Fay in the animated film, Monster Family.

In 2018, she starred as Elizabeth McKenna in The Guernsey Literary and Potato Peel Pie Society.

In May 2019, it was announced that Brown Findlay would star in the role of Lenina Crowne in the NBCUniversal series Brave New World, based on the classic 1932 novel by Aldous Huxley. The series was subsequently moved to Peacock network.