Mr Kenny, 59, could not be revived despite the best efforts of doctors and nurses.
It's believed Mr Kenny first visited the hospital yesterday for a health check and returned, driving himself there, today.
Soon after arriving at the hospital this morning, Mr Kenny suffered what's believed to be a burst blood vessel.
''People all over Phuket will be distressed to hear the news,'' Phuketwan editor Alan Morison said today. ''Brad was just a wonderful guy who gave and gave in so many ways.
''He was a stalwart of Patong Rotary and the Phuket Navy League. He gave his time and his cash over and over and with complete and enduring generosity.''
There were times when Rohingya women and children, in a family shelter north of Phuket, had no government budget to pay for food.
Brad Kenny would conduct a whip-around of Rotary members, buy the food and deliver it in person.
The sea gypsies off the coast of Ranong lived in appalling squalor. Brad Kenny organised a series of projects to give them basic sanitation and a water supply that turned their lives around.
After the Phuket tsunami, while many people were content to get their own lives back in order, Brad Kenny put his own life on-hold to help hundreds of others first.
''He was a man for whom we all had the utmost admiration,'' Morison said. ''This is a devastating blow to his family.
''The underprivileged of the whole Andaman coast were also part of Brad's family. He simply cannot be replaced.''
Denny Bowman, a fellow American and an old friend, said: ''Brad was just such a worthwhile, honest, giving guy.
''There are so many people who know him and have interacted with him at some time. He reached out to people from school children through to key administrators.
''Endless numbers of people were touched by Brad Kenny.''
Mr Kenny settled on Phuket about 20 years ago and established a successful swimming pool and engineering business. He leaves a wife, Na, and three children.
ROTARY Club of Patong Beach (RCoPB) President Walter Wyler today paid tribute to immediate past president and current district assistant governor Brad Kenny, who unexpectedly passed away this morning.
''Brad was a stalwart of RCoPB who gave his time, and often his own money, unstintingly on many occasions to help those in need in the Phuket and Phang Na communities. His relentless energy and enthusiasm for Rotary and the many charitable projects he was involved in was an inspiration to all members of our club.
''Brad was always on the lookout for those in our society who needed a helping hand. He was particularly a great friend of the Rohingya and sea gypsies in Ranong, using his expertise in sanitation and water supply to help turn lives around.
''His presidency last year was marked by a driving support for a number of charities Brad brought his energy to focus on.
'''Brad will not only be greatly missed by all members of RCoPB, but also the wider Rotary community on Phuket and beyond. On behalf of all members of our club, I express my condolences to his wife Na and three children,'' Mr Wyler said.
Mr Kenny was president last year when RCoPB received Rotary Thailand's highest service award, the HRH Crown Prince Maha Vajiralongkorn Trophy, bestowed jointly with the Rotary Club of Nakhom Pathom for three projects that helped the children of Burmese migrant workers and sea gypsies.
Mr Kenny was also heavily involved with Youthlinc, which focused on improving facilities at the Burmese learning centre in Kuraburi, Phang Nga.
Brad Kenny was a longtime member of RCoPB. He ran his business Environmental Solutions & Protection (ESP) Corporation on Phuket since 1999.
He was also very active in the Navy League, receiving the prestigious Sahataya Naval Insignia and Certificate from the Royal Thai Navy in 2010.
He served as the US Embassy in Bangkok's Naval Attache, as the point of contact and coordinator for visiting US naval ships to Phuket.
Contact: Walter Wyler, President. Email: email@example.com. Tel: +66 (0) 0900152289 or go to Facebook: @rotaryclubofpatongbeach; website www.rotarypatong.org
Brad Kenny helped Jirawat Kuramakanok to make this 10-years tsunami anniversary special about Patong Rotary's work from 2004. WARNING: This video contains extremely graphic images. But as Brad would say: ''That's life.'' He is seen encouraging a boy with cerebral palsy to walk and helping with rebuilding.