A vicious mob killer emptied his gun into an NYPD cop under orders from a Colombo crime family boss, he told jurors in Brooklyn federal court Thursday.

Former Colombo consigliere Joel Cacace is on trial for murder for allegedly ordering the 1997 slaying of Police Officer Ralph Dols because he’d married Cacace’s ex-wife.

One triggerman, Dino Calabro, told rapt jurors he and a colleague walked up to Dols around midnight in front of his Brooklyn home.

The unsuspecting cop said, “What’s up?” before the men opened fire, Calabro said.

“I just emptied my gun,” Calabro said, noting that he saw Dols slumped over the hood of his Oldsmobile Cutlass Supreme after being riddled with bullets.