Complex sentence: Polley's career at Zenith included positions as a product engineer and mechanical engineer; he later managed its Video Recording Group, the Advanced Mechanical Design Group; he eventually became an Assistant Division Chief for Zenth's Mechanical Engineering Group.
Split into simpler sentences: Polley's career at Zenith included positions as a product engineer and mechanical engineer. Polley later managed its Video Recording Group, then its Advanced Mechanical Design Group; he eventually became Assistant Division Chief for Zenth's Mechanical Engineering Group.