The Australian-born singer, 35, had finished eighth on Season 7 of the singing competition in 2008.
"It is with a heavy heart and inconsolable sadness that we confirm the passing of our friend Michael Johns," the administrator of his website Tweeted on Saturday.
"Our family is devastated by the passing of our beloved Michael, a wonderful husband, son, brother, uncle, and friend," his family said in a statement. "We appreciate the outpouring of love and support we have received from all corners of the globe. Please keep Michael in your thoughts and respect our need to grieve privately in this difficult time."
"A truly great guy. Rest in peace Michael," Tweeted former Idol judge Simon Cowell, while Randy Jackson's message was "So Sad. Too soon. Thank you for sharing your talents with us." Paula Abdul sent her heartfelt condolences to his family.
Johns is the first American Idol finalist to pass away. Born Michael Lee, he came to the U.S. on a tennis scholarship in 1997 and moved to Atlanta to pursue a music career before auditioning for Idol, the Atlanta Journal-Constitution reports.
After his shock elimination from the show, he wrote songs for films, documentaries and other artists. Johns released a holiday song called "Another Christmas" in 2008, with proceeds going towards the fight against Lou Gehrig's disease.
He released an EP Love & Sex in 2012, and Tweeted on July 10 this year that he was headed back to the studio to work on a new song, called "Reckless."
"Michael Johns was an incredible talent and we are deeply saddened by the news of his passing," the FOX network and Idol producers Fremantle Media North America and 19 Entertainment said in a statement on Saturday. "He was a part of our American Idol family and he will be truly missed. Our hearts and prayers go out to his family and friends during this difficult time."
Johns' death is being mourned by fans and American Idol alums, including Melinda Doolittle, who wrote on Instagram, "I can't express how sad I am right now. Michael Johns, rest in peace, my friend. You will be greatly missed."
Real Housewives of Beverly Hills star Yolanda Foster (whose husband, music producer David, worked with Johns), Tweeted, "R.I.P our beautiful friend @michael_johns You will live in our hearts forever @AmericanIdol."
Listen to Michael John's Christmas song here:
It is with a heavy heart and inconsolable sadness that we confirm the passing of our friend Michael Johns. No other details will b avail at— Sedz (@SedzOz) August 2, 2014
this time.Please respect the privacy of Mike's family and friends as we know u will.Members and admin of @mjohnsonline pass on their prayers— Sedz (@SedzOz) August 2, 2014