[3] Grzelcyk attended Belmont Hill School before joining the USNTDP of the USHL, winning a gold medal at the 2012 U18 World Juniors in the Czech Republic.

He scored the overtime winning goal in the 63rd Beanpot that gave BU their 30th title, and was named the tournament MVP.

He joined Boston's affiliate Providence Bruins of the American Hockey League (AHL) on an Amateur Tryout Agreement for the rest of the 2015–16 campaign.

On October 25, 2021 Grzelcyk's father was diagnosed with prostate cancer, while undergoing radiation treatments John Sr. continued work at TD Garden.

[17] The two elder Grzelcyks are members of the arena's "bull gang", which converts the building from hockey to basketball and back: John Jr. is also a Zamboni driver for certain Bruins games.