Horwill started his Super Rugby career with the Queensland Reds, playing 116 matches and captaining the side to the 2011 Super Rugby championship.
The lock, nicknamed Kev, made his Wallabies debut in 2007 and narrowly missed out on that year's World Cup squa.d
His time would come four years, captaining the Wallabies in the 2011 Rugby World Cup, capping off a dream 2011 season.
Horwill was a surprise omission from the 2015 Rugby World Cup squad and the Queensland veteran made the move to English club Harlequins at the end of that season.
He returned to Australia under the Giteau Law in the 2016 England series and played the last of his 62 Tests at Suncorp Stadium in the first game of that series.
The 33-year-old played four seasons at Harlequins, captaining the side in recent seasons.
Horwill announced his retirement on Monday night, thanking fans, teammates, coaches and his family for their support.
"The time has come for me to retire from professional rugby. I have given this great game everything I have & I feel I can no longer give what is needed to play at this elite level," he wrote.
"Rugby has been a big part of my life for many years, never in my wildest dreams would I have envisaged the incredible journey it has taken me on.
I’ve had the privilege and honour to not only play, but captain my state, country and club on a number of occasions over my 14-year career."
Horwill listed that 2011 championship and leading the Wallabies as among his career highlights.
"Winning the Super Rugby title and being asked to captain my country are some of the moments I’ll cherish for a lifetime," he wrote.
"I want to thank all the coaches and staff I’ve worked with over my career, you have all made an impact and shaped me into the player I am today."
The Brisbane-born and bred Horwill had his fair share of injury setbacks, including a gruesome hamstring injury that cost him a season in 2012 and he was quick to thank medical staff for the role they played.
"Special thanks must go to the physios and doctors, who have put me back together. We spent more time together than I would have liked, but without your care, I wouldn’t have played as long," he wrote.
"To all the players I have had the pleasure to play with (and against), you are why I have loved my career so much. We have made some unforgettable memories and I know I have friends for life.
"To the fans all over the world,who have supported me throughout my career, thank you .
"My job would not have existed without you; you are what makes this game so special."
Horwill will play on with Harlequins for the rest of the English season.