Sofia Vassilieva was born on October 22, 1992, in Minneapolis, Minnesota, to Russian immigrant parents, Larissa and Vladimir Vassiliev. She started acting at a young age, winning the title of Child Actress and first runner-up Child Model of the Year at the International Modeling and Talent Association (IMTA) in New York when she was just seven years old.

Her first role came shortly after, playing the granddaughter of a retired Russian general in an episode of the CBS television series "The Agency".

Her early career included roles in "The Brady Bunch in the White House" and "Inhabited," but she gained widespread recognition for her portrayal of Eloise in the made-for-television movies "Eloise at the Plaza" and "Eloise at Christmastime".

Her performance as Eloise earned her a nomination for the Best Leading Young Actress in a TV Movie, Miniseries, or Special at the 2004 Young Artist Awards. In 2004, Sofia Vassilieva was cast as Ariel DuBois in the Emmy-winning TV series "Medium," where she played the daughter of Patricia Arquette's character, Allison DuBois, for seven seasons.

Her role in "Medium" won her a Young Artist Award for Best Supporting Actress in a TV Drama in 2006. One of her most notable roles came in 2009 when she played the terminally ill teenager Kate Fitzgerald in the film adaptation of Jodi Picoult's novel "My Sister's Keeper."

For this role, she shaved her head and eyebrows, and her performance earned her another Young Artist Award for Best Supporting Actress in a Feature Film.

She continued to pursue her education while acting, attending Columbia University in New York City, where she graduated with a Bachelor of Arts and Science degree. She has also taken acting courses at the Royal Academy of Dramatic Art in London and has been involved in various activities such as ballet, horse riding, and yoga.

In addition to her acting career, Sofia Vassilieva has lent her voice to video games and has been involved in various charitable activities. She became an honorary ambassador for Stand Up to Cancer in 2009 and has continued to support various causes throughout her career.

Her recent work includes roles in "The Little Things" (2021), "Looking for Alaska" (2019), and "Lucifer" (2021). She continues to be active in the entertainment industry, balancing her career with her personal interests and commitments.