A "compulsive liar" is a person who cannot stop themselves from telling lies, even when they don't need to. They lie very frequently and it becomes a habit they can't control. Full definition
An almost compulsive liar, she can sometimes be quite the opposite of charming. (seventh-row.com)
Portman's Sam is a self - professed compulsive liar who meets cute with Braff's character in a doctor's office waiting room and becomes his impromptu sidekick. (christylemire.com)