Sulkin made his UK acting debut on the Working Title Comedy Sixty Six in 2004 as the lead Bernie Rubens alongside Helena Bonham Carter, Eddie Marsan and Catherine Tate. Sulkin also played the role of JJ in the Disney Channel comedy As the Bell Rings, worked on a CBBC children sci-fi show The Sarah Jane Adventures (spin-off of Doctor Who) playing Adam in series 3, a two-episode story The Mad Woman in the Attic.
Sulkin’s time on Disney Channel continued, starring as himself in Pass the Plate representing the UK.
He then appeared in 30 episodes of the Emmy-Award winning Disney Channel series Wizards of Waverly Place where he played Alex's (Selena Gomez) love interest Mason Greyback. Gregg reprised his role in season 3 and season 4 and through to its finale episode. He continued his role as Mason Greyback in The Wizards Return: Alex vs. Alex on Disney Channel.
After that, he filmed in New Zealand making the Disney Channel film Avalon High.
In 2012, Sulkin starred in the American hit teen drama series Pretty Little Liars as a recurring character, Wesley Fitzgerald, brother of Ezra Fitz.
Since 2014, Sulkin has starred as a series regular in the MTV comedy Faking It portraying the character of Liam Booker.
He has also appeared on TV shows, such as “Young & Hungry” & “Melissa & Joey” for Freeform and “Life in Pieces” on CBS
Sulkin defeated Victoria Justice on Spike TV's Lip Sync Battle where he wore a wig and went shirtless to perform the hit Kelis song "Milkshake."
Sulkin starred in the leading role of Sam Fuller in the horror film Don't Hang Up, which was released in theaters on February 10, 2017.
Most recently, Gregg was cast as Chase Stein in Marvel’s Runaways. The pilot co-stars James Marsters as his father and Ever Carridine as his mother. The show was shot for Hulu and is currently awaiting a pick up.
Pretty Little Liars • Marvel's Runaways