Grendys was born in Chicago in the early 1960s. His Father was a butcher who was also involved in the wholesale and Retail meat trade. His grandparents emigrated to the US from Poland.

Grendys started full-time at Koch Foods in the mid-1980s, when it had just 13 employees. The owner Fred Koch was one of his father's customers, having started Koch Foods in 1973, and offered him a 50% equity share in the Business.

He was educated at Holy Cross High School in River Grove, and worked at Koch Foods part-time while a student. In 1984, he received a bachelor's degree in Finance and Marketing from Loyola University.

He bought out Fred Koch in 1992. Grendys' companies now slaughter, ship and sell chicken using the Koch Foods, Antioch Farms, Preferred Foods and Rogers Royal brands. Koch slaughters over 12 million chickens and processes over 50,000,000 pounds (23,000 t) of chicken every week.

In 2011, Koch employed 14,000 people, had an annual turnover of $2.8 billion, and was one of America's five largest "fully-integrated chicken processors".

In a 2014 interview, Grendys stated that he planned to acquire more companies and that he might branch out into another protein, but that he was "not sure what that protein would be yet". Rather than running a chicken company, he sees it as "the Business of converting corn and soybeans into meat protein".

As of February 2018, Forbes estimated his net worth at US$2.9 billion.