Sebastian was born in the Romanian city of Constanta on August 13th of 1982. Then, at eight years old, he and his mother moved to Vienna, Austria where she was employed as a pianist. Four years later at the age of twelve, his mother moved with him to Rockland County, New York after she married the headmaster of an American private school.

During his years at the Rockland Country Day School, he starred in productions such as Harvey, Cyrano de Bergerac, Little Shop of Horrors, Over Here, and West Side Story. He also attended the Stagedoor Manor summer camp where he was introduced to acting and was cast in many camp productions. It was then that he decided to take acting seriously and began to apply for acting programs at several universities.

He was accepted to and attended Rutgers University’s Mason Gross School of the Arts. This gave him the opportunity to spend a year abroad studying acting at Shakespeare’s Globe Theatre in London, England.

After returning from England, Sebastian continued attending Rutgers and was cast in small roles in independent films like Tony n’ Tina’s Wedding and Red Doors. He was also cast in a guest starring role on Law & Order as “Justin Capshaw”, a boy who was accused of murder and had been kidnapped and emotionally abused by his captor. Immediately after graduating from Rutgers in 2005, Sebastian was cast in The Architect. That same year he portrayed The Covenant. He was cast in Eric Bogosian’s play Talk Radio as “Kent” in 2007 where he had a chance to work with veteran Broadway actor, Liev Schreiber.

In 2007, Sebastian first appeared as St. Jude’s graduate Carter Baizen on the soon to be hit CW television series Gossip Girl. He later reprised the role in several episodes throughout the show’s second and third seasons.

Sebastian also appeared in The Education of Charlie Banks as Leo in 2007, which premiered at the Tribeca Film Festival in April and won an award for “Best New York Narrative”. He then went to star in Rachel Getting Married alongside Anne Hathaway in 2008, as well as Spread with Ashton Kutcher, a sex comedy centered on a serial womanizer and his jilted lover in 2009.

He was then cast in the NBC drama Kings, where he portrayed Prince Jack Benjamin, the closeted gay eldest son of King Silas Benjamin and an analogue to the biblical Jonathan. Albeit the series was popular among the views, it was later canceled before the final episodes aired.

In 2010, he had a part alongside Natalie Portman and Mila Kunis in Darren Aronofsky’s ballet thriller, Black Swan. Sebastian also played a character named Blaine, the main antagonist in the Hot Tub Time Machine.

Then in 2012 Sebastian caught the attention of a much wider fan base as he was tapped to play James Buchanan Barnes, better known as simply Bucky in the film Captain America: The First Avenger, based on the Marvel Comics character, alongside Chris Evans, who played Captain America. Sebastian is said to reprise the role in the sequel Captain America: The Winter Soldier set for a April 2014 release.

He also starred alongside Amanda Seyfried in the thriller Gone, the USA Network Original series Political Animals as T.J. Hammond, a troubled thirty year old son of two politician parents starring Sigourney Weaver, Carla Gugino, Ellen Burstyn, Ciarán Hinds and James Wolk also in 2012.

Sebastian is also set to appear in the supernatural horror film The Apparition, which co-stars Ashley Greene, in August of 2012.