The six-time British champion enjoyed one of the most successful -- and colorful -- careers in horse racing.
Nicknamed "King Kieren", Fallon rode 16 British Classic winners, including three victories in the English Derby at Epsom. He also landed six Irish Classics and a host of other Group 1 races around the world, including two wins in the prestigious Prix de l'Arc de Triomphe in France.
Former Northern Irish jump jockey AP McCoy, a record 20-time champion, tweeted that Fallon was "one of the most naturally gifted jockeys I've ever seen."
Three-time champion Frankie Dettori, who has battled against Fallon in Europe's top races, paid tribute to the Irishman's fierce competitive nature.
"I've known and ridden alongside Kieren for the past 30 years," the Italian told the Racing Post.
"He's always been at the top of my list as the most ferocious competitor, and over the years we've gained great respect for each other. It's sad to see Kieren finish and I wish him all the best."
British-Barbadian flat-racing trainer Sir Michael Stoute said "Kieren had a power that could propel horses to go faster than they thought they could."
Despite the success, Fallon's career has also been dogged by controversy. In 1994 he was banned for six months for hauling fellow jockey Stuart Webster off his horse after a racing incident. In 2007 he was tried for race fixing and banned for 18 months in a case that was later dismissed.
The Irishman also served two drugs bans after testing positive for recreational drugs.
Noel Hayes, director of betting company Sportsbook, tweeted that Fallon's retirement was "a timely check to those who wish to judge a man without understanding the road they walk."
Trainer Michael O'Callaghan, who Fallon had been riding for this season, tweeted a heartfelt tribute to the celebrated jockey, calling him a "childhood hero".
The star's retirement was announced by the Irish Turf Club
's chief medical officer, Dr Adrian McGoldrick, who told CNN that his depression was "significant" and appeared to have been ongoing for three years.
"I don't know [why it wasn't detected sooner]," said McGoldrick, who noticed it when Fallon came to him for his jockey's licence in April this year.
"He had symptoms of profound, ongoing fatigue for years and had been fully investigated in America. But he may not have admitted it at the time. There's a major stigma attached to depression -- a lot of people will not open up, certainly in Great Britain and Ireland. People will talk about anything bar depression."
He added that the trauma of Fallon's race-fixing court case in 2007 was "potentially" a trigger for the problem -- the jockey told McGoldrick it had "broken him financially and mentally" -- however his symptoms suggested the issue developed "more so in the last three years".
Fallon will be admitted to a hospital in Dublin for treatment.
'Extremely difficult job'
The announcement has put increased focus on mental health issues within horse racing, which McGoldrick said are "much more common than in any other [sporting] elite."
In an unpublished survey in Ireland last year, 49% out of 122 Irish jockeys reported depressive symptoms, the doctor said. This compares to one in four
of all UK adults.
"It's an extremely difficult job," said McGoldrick. "It's difficult making weight, they work very long hours, there are insecurities in the job. I think it's one of the most difficult professions on earth."
In January 2015, 23-year-old Irish jockey Mark Enright admitted to suffering depression and spent three weeks away from racing to help tackle the issue. "We're in a sport where depression is rampant," he told the Racing Post
A high incidence of mental health issues is also well-documented in football -- last year the international players' federation, FifPro, published new research showing
that 38% of 607 current players reported symptoms of depression.
In racing, a 24-hour support line for anyone involved in the sport will be launched by Horse Racing Ireland at the Galway Races later this month, while the Professional Jockeys Association and Racing Welfare
, an industry charity, already have similar hotlines.
"The Racing industry provides substantial support to its participants in terms of well being and mental health," said Britain's Horse Racing Association
in a statement.
"As well as the helpline, PJA members also receive six free face-to-face therapy sessions, in addition to support from the Injured Jockeys Fund. The BHA has also recently recruited a dedicated Welfare Development Manager with a remit for delivering a participant welfare strategy."
The BHA and Racing Welfare have recently joined the PJA in signing the UK's Mental Health Charter for Sport and Recreation
, a collective agreement that aims to tackle mental health issues in sport.
Natalie Fallon, one of Kieren's three daughters, posted a heartfelt tribute to her dad on Instagram, saying she "couldn't be prouder" of the man who has taught her "to always follow my ambitions and dreams and never give up."
The retired jockey tweeted he was "overwhelmed" with the supportive response.