Crennel announced Monday morning that he is retiring.
"Football has been my entire life and it's been a dream come true to coach for 50 years," said Crennel. "My goal was to put every player and coach in the best position to succeed and I consider every guy I coached or worked with a part of my family."
Crennel's most recent roles in the league were with the Texans, spending the last eight years with the team.
His tenure in Houston started in 2014, where he served as defensive coordinator, then assistant head coach, associate head coach, interim head coach, and in 2021, senior advisor for football performance.
As interim head coach of the Texans in 2020, Crennel became the oldest person to serve as a head coach in a game at 73 years old.
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Prior to the Texans, Crennel's journey took him to the New York Giants, New England Patriots, New York Jets, Cleveland Browns, and the Kansas City Chiefs.
He's collected Super Bowl wins as well with the Giants and Patriots.
And it was with the Giants where it all began for Crennel in 1981.
But his coaching career took off even earlier than that. It kicked off at his alma mater in Western Kentucky where he was a graduate assistant in 1970.
On Monday, Crennel received praise from members of the media, fans and his former players, including the Arizona Cardinals' J.J. Watt.
Watt worked under Crennel when the star defensive end was in Houston.
"Absolute legend of the game and even better man," said Watt.
"He's a first-class guy respected by everyone who's known him. It was an honor to cover him in his eight seasons with the Texans," said John McClain, now-retired longtime Houston Chronicle writer.
"During several of the most chaotic years in Texans history, he was always a pro's pro and took the time to say hello, answer a Q, etc.," said Houston Chronicle columnist Brian T. Smith.