A jury on Friday found 29-year-old Iftekhar Murtaza guilty of two counts of murder, one count of attempted murder and one count of conspiracy.
Prosecutors are seeking the death penalty.
They say Murtaza killed the relatives of Shayona Dhanak, who was a college freshman in 2007, after she blamed the couple's breakup on her Hindu family's opposition to her dating a Muslim.
The family's home was torched and Dhanak's mother left for dead. The bodies of Dhanak's father and sister were found in a park the next day.