He's one of the longest consecutively tenured by a single team. There are some coaches who have coached longer, but not consecutively, or not for the same team, but Tomlin was the head coach for the Steelers for 19 years. And in those 19 years, he never had a losing season.
He had one more year on his current contract when he decided to retire.
He was mostly known for being able to wrangle the more "unique" personalities of the NFL and for his answers during press conferences. Giving us such gems as "The standard is the standard", "and things of that nature", "We're still squirreling those nuts", "We do not live in our fears", etc.