Howard, who last played in the major leagues during the 2016 season, confirmed the decision in an essay entitled "Thank You, Philly" for The Players' Tribune.
"And that's definitely something I've learned, one way or another, over these last 14 years: How, in baseball, man…you truly do have to take the all of it," he wrote. "You truly don't get your share of the sweet, in this game, without taking your share of the sour."
The Phillies selected Howard in the fifth round of the 2001 MLB draft. He made a quick ascent through the club's minor league system before making his debut late in the 2004 campaign, when he flashed his power potential with two home runs in 39 at-bats.
He captured National League Rookie of the Year honors in 2005 after posting a .288/.356/.567 triple-slash line with 22 homers across 88 appearances.
One year later, he was named the 2006 NL MVP after racking up an MLB-high 58 home runs and accumulating a career-best 1.084 OPS in 159 games for Philadelphia.
Howard, who spent time in Triple-A for the Atlanta Braves and Colorado Rockies in 2017, also features a 2008 World Series title, three All-Star Game selections and a Silver Slugger Award on his resume.
"It's been a wild ride…and I'm glad that I got to stay on it for as long as I did," Howard wrote. "Which I guess has really also kind of become my overall perspective on things: How, when it's come to these last 14 years of mine—nothing has ever been easy for long, and nothing has ever been perfect for long.
"But I wouldn't have it any other way."
He finishes his career with 382 home runs—the second-most in Phillies history behind only fellow team legend Mike Schmidt (548)—1,194 RBI and 848 runs scored in 1,572 MLB games.