And on Tuesday, Johnson was elected to the Hall of Fame, the culmination of a career that included five Cy Young awards and 303 career wins. Johnson's 4,875 career strikeouts are the second-most in major league history behind Nolan Ryan.
"I really got to see him evolve from those days in Jamestown right out of college to the dominant pitcher he came to be," said Showalter, who managed Johnson in 1999 and 2000, both years in which Johnson won the Cy Young.
"He had some control issues early, but he really evolved once he got to Seattle. It's such a hard thing, being 6-10 and throwing from way out there from the release point he had. With him, you really had to throw out the blueprint. He was a break from anything you ever had in terms of managing and dealing with a pitcher."
Johnson received 97.3 percent of the vote in his first year on the Hall of Fame ballot, part of a four-player class that included three pitchers elected by the Baseball Writers' Association of America for the first time in history.
Pitchers Pedro Martinez (91.1 percent of the vote) and John Smoltz (82.9), as well as second baseman Craig Biggio (82.7), were also voted into the Hall of Fame this year.
"When he was on the mound, you knew you only had to score a couple of runs," Showalter said about Johnson. "People really forget about his slider. People threw as hard as he did, but you couldn't re-create his release point, coming from left side and being 6-10. That's a challenge.
"It's a tribute to the kind of athlete he was to be able to hold that together for as long as he was able to. It's so hard to do for a 6-3 guy, let alone a 6-10 guy."