The jury found Fujita, 20, guilty of first-degree murder in the stabbing and strangulation of the 18-year-old Astley at his home in July 2011.
A first-degree murder conviction carries an automatic sentence of life without parole.
Fujita will be formally sentenced Thursday afternoon.
The Middlesex Superior court jury of four women and eight men needed less than two days to determine the fate of Fujita.
Fujita hung his head as the jury foreman read the verdict, while Astley's mother and Fujita's parents broke down in tears, the station says.
In closing arguments Tuesday, defense attorney William Sullivan said Fujita was not in his right mind when he killed Astley. Sullivan cited a family history of mental illness, signs of depression in the year leading up to the murder and even raised the possibility that Fujita suffered brain injuries from playing high school football.
Prosecutor Lisa McGovern argued that the murder was carefully planned and fueled by rage over a breakup. She reminded jurors that Fujita asked Astley to park her car away from his house that night, stashed his bloody clothes and left her body in a nearby marsh.
McGovern said there is no such thing as a "psychosis fairy."
Jurors had 240 pieces of evidence and three weeks of written testimony to review during their deliberations.