Saint John was born to Rev. Dr. Appianda Arthur and Aba Arthur (née Enim) in the United States but moved to Ghana at 6 months old. The family moved to Colorado Springs when she was 12. Her father was a clarinet player and member of the Ghanaian army before emigrating to the United States to attend graduate school. Saint John cites her father as her biggest inspiration. In 1999, she graduated from Wesleyan University with an English major. Her father received a PhD in ethnomusicology in 1977, also from Wesleyan.
Saint John was married to Peter Saint John until his death in 2013. They had one daughter, Lael Saint John.
Saint John led PepsiCo's foray into music festival-based marketing as head of the Music and Entertainment Marketing; the division itself came from her suggestion that the company sponsor music festivals and award shows. She remained with the company for almost a decade before joining Beats Music in 2014; when Jimmy Iovine of Beats personally recruited Saint John based on her experience in music marketing, Saint John moved from New York to Los Angeles to lead marketing for the company.
Beats was purchased shortly thereafter by Apple and Saint John became the head of global consumer marketing for iTunes and Apple Music, commuting between Los Angeles and Apple's Cupertino headquarters several times a week. Journalists described Saint John's presentation of the redesigned Apple Music onstage during the 2016 Apple Worldwide Developers Conference as her breakthrough moment, with a captivating "passion" and persona that captivated both local and online audiences. Biz Carson of Business Insider said that her appearance helped to emphasize Apple's complete rebuild of its Apple Music software. For instance, she attempted to have the audience sing along with on-screen lyrics, a feature in the redesigned app. Buzzfeed wrote that she was the "coolest" person to ever take the stage at an Apple keynote.
On June 6, 2017, Saint John became Chief Brand Officer at Uber. She has been given the task of turning Uber into a brand that people love as much as Apple.
After college, Saint John worked at the advertising agencies Arnold Worldwide and Spike Lee’s Spike DDB, as well as the fashion brand Ashley Stewart, where she was Vice President of Marketing.