Los Angeles County Superior Court Judge Cynthia Ulfig sentenced Dykstra after refusing to allow him to withdraw a no-contest plea.
The judge said the theft scheme showed sophistication and extensive planning.
Dykstra was immediately remanded to custody and he walked into the court's back room, hands in his pockets.
Dykstra, 49, initially pleaded not guilty to 25 counts after police arrested him and found cocaine, Ecstasy and synthetic human growth hormone at his Los Angeles home last April.
Prosecutors said Dykstra and two co-defendants tried to lease and then sell high-end cars from several car dealerships by claiming credit through a phony business. His accountant, Robert Hymers, pleaded no contest to one count of identity theft, while Christopher Gavanis pleaded no contest to one count of filing a false financial statement. They are both awaiting sentencing.
Dykstra changed his plea in October to no contest and in exchange prosecutors dropped 21 counts.
Dykstra has had a series of legal problems over the past year. He faces federal bankruptcy charges and is scheduled to stand trial this summer.
Dykstra, who bought a mansion once owned by hockey star Wayne Gretzky, filed for bankruptcy three years ago, claiming he owed more than $31 million and had only $50,000 in assets. Federal prosecutors said that after filing, Dykstra hid, sold or destroyed more than $400,000 worth of items from the $18.5 million mansion without permission of a bankruptcy trustee.
Dykstra also has pleaded not guilty to indecent exposure charges for allegedly exposing himself to women he met on Craigslist.
The ex-major leaguer has been in a sober living facility, according to court documents.