Sulkin was born in London. He was raised Jewish and had his Bar Mitzvah by the Western Wall in Israel. He attended Highgate School in North London.
Sulkin made his acting debut in the 2002 mini-series Doctor Zhivago. Since then, a majority of Sulkin's work has been with Disney. He played the role of JJ in the Disney Channel comedy, As The Bell Rings from 2007 to 2008 and Pass the Plate as Gregg from the UK starting in those same years. In 2010, Gregg saw a major breakthrough by playing a recurring guest role on the Disney Channel series Wizards of Waverly Place, where he played Alex's love interest Mason Greyback. He reprised his role in 4 episodes of season 3 and returned to the series in its fourth season, and through to its finale. Sulkin also reprised the role for the TV movie The Wizards Return: Alex vs. Alex. In 2010 he went to New Zealand making a Disney Channel film Avalon High, which premiered on November 12, 2010.
In 2012, Gregg was starred in the American teen drama series Pretty Little Liars as a recurring character, Wesley Fitzgerald brother of Ezra Fitz.
- He is a big fan of soccer and supports the team Arsenal.
- He was rumoured to be dating Australian actress Adelaide Kane who played Cora Hale on the MTV series Teen Wolf.