Hajjar's family fled Lebanon during the Civil War, moving to the United States and settling in El Paso, Texas. When Hajjar was fourteen his mother died from cancer and his father, who had been distant, left the family. His brother, only 18 years old at the time, assumed responsibility for the family and raised Tony and his sister.
Hajjar joined post-hardcore band At The Drive-In in 1997, becoming the band's fourth drummer. He remained with the band until they split up in 2001. During breaks from touring Hajjar taught chemistry. Following the demise of At The Drive-In Hajjar formed the band Sparta with fellow ATD-I members Paul Hinojos and Jim Ward. He also played on several other bands' recordings, including Jimmy Eat World's "Disintegration" on their "Stay on My Side Tonight" EP. As a drummer, his technique is particularly unusual. He plays a right-handed set up, playing right-handed, but switching to his naturally more dominant left hand during fills and rolls.
In 2006 Hajjar released a 16-minute short film entitled "Eme Nakia" (Arabic for Mother Nakia, Hajjar's mother's name) as part of a special edition of Sparta's album, Threes following Hajjar's early life story.
Currently Tony Hajjar is in Sparta and At the Drive-In.