Complex sentence: Leonard Norman Stern is the Chairman and CEO of the privately owned Hartz Group based in New York City, and oversees operations of the extensive Hartz real estate portfolio owned and operated under its Hartz Mountain Industries subsidiary company, of which is is also Chairman and CEO.
Split into simpler sentences: Leonard Norman Stern is the Chairman and CEO of the privately owned Hartz Group based in New York City. Additionally, he oversees operations of the extensive Hartz real estate portfolio owned and operated under its Hartz Mountain Industries subsidiary company, of which is is also Chairman and CEO.