"I've never really talked about this, but I would go days without eating," the Cosmopolitan cover girl said in the September issue.
"Or maybe I'd have some fruit and then go to the gym for three hours."
The 23-year-old had bit parts in The O.C. and How I Met Your Mother and a lead role in the short-lived series Privileged before landing the role of Aria on the hit ABC Family show Pretty Little Liars in 2010.
"I knew I had a problem," Lucy said of her eating disorder.
"It was a gradual process but I changed myself."
To read Lucy's entire interview, pick up the new issue of Cosmopolitan on newsstands August 14.