Detectives are searching for 18-year-old Tyler Rodgers, who is currently on probation for robbery.
The incident took place Thursday afternoon, and much of it was caught on surveillance camera.
At about 4 p.m., a suspect who fits Rodgers' description walked into the Cigar and Smoke Shop at the intersection of Artesia and Aviation boulevards.
In the footage, the suspect is seen carrying a long cardboard box. He suddenly pulls out a shotgun from that box, points it at the clerk and ushers him to the back of the store. Police said the suspect handcuffed the clerk, took money from the register and then slit the employee's throat. That clerk was rushed to emergency surgery and was expected to recover.
The suspect was described as a young black man with a thin build, light mustache and short hair. If caught, the suspect would be charged with robbery and attempted murder.
Anyone with information is urged to call Manhattan Beach police at (310) 802-5170.