A suspect was arrested early Wednesday in the slaying of a green-cab driver in The Bronx who was on the phone with his brother-in-law and asked for help moments before being gunned down.

Nicholas Sanchez, 24, was busted on murder and robbery charges in the death of Barry Mamadou, cops said.

Just before being shot Monday, Mamadou, the first green-cab driver killed on the job, told his brother-in-law that a passenger had a gun.

Mamadou’s body was found at 1:30 a.m. in the driver’s seat of his Lincoln Town Car at East 189th Street and Beaumont Avenue.

He leaves behind three kids and his pregnant wife, Kadiatou Diallo.

Sanchez was grilled by cops at the 48th Precinct station house and was transferred to Central Booking late Wednesday.