Thailand aimed to claim 121 gold medals and win back the overall title at the SEA Games in the Philippines but could only claim 92 titles.
The Thais finished third in the medal standings behind the hosts (149) and Vietnam (98).
The Thai men's U23 football side, who were chasing their fourth consecutive crown, were embarrassingly eliminated in the first round.
The senior men's football squad also had an unconvincing spell in the World Cup qualifiers, and the War Elephants will resume their campaign in March.
The women's football team reached the final only to lose to Vietnam who are now the most successful nation in the discipline at the biennial event with five gold medals, one more than Thailand's tally.
The Thais also were defeated by the same opponents in the AFF Women's Championship on home soil in Chon Buri.
At the 2019 Fifa Women's World Cup, the Chaba Kaew suffered three heavy defeats.
Having set a three gold medal target, the Thai tennis team failed to win a single title in the Philippines.
On the bright side, golfer Jazz Janewattananond won four tournaments on the Asian Tour while taekwondo star Panipak Wongpattanakit was a winner at the world championships.
Other successful Thai athletes included mixed doubles badminton duo Dechapol Puavaranukroh and Sapsiree Taerattanachai, who won three titles, and footballer Theerathon Bunmathan, who became the first Thai to win the Japanese league title.
Badminton star Kunlavut Vitidsarn won his third world junior title to become the first male player to achieve the feat.
Teenage amateur golfer Atthaya Thitikul, 16, continued her rise with her second triumph at the Ladies European Thailand Championship in Pattaya.
But Jazz stood out from the rest and is named the Bangkok Post Athlete of the Year.
The 24-year-old ended the 2019 season with back-to-backs wins at the recent Indonesian Masters and Thailand Masters, the Asian Tour's last two events of the year.
Jazz, whose real name is Atiwit, also won in Singapore and South Korea earlier in the season.
Jazz is the only second player in Asian Tour history to win four events in one season, following in the footsteps of compatriot Thaworn Wiratchant who achieved the feat in 2005.
With his success at the Thailand Masters, Jazz surpassed US$1 million in earnings on the Asian Tour this season.
He is the fifth player to cross the $1 million mark in one season after Jeev Milkha Singh (2008), Kiradech Aphibarnrat (2013), Anirban Lahiri (2015) and Scott Hend (2016).
One of the busiest golfers in the world, Jazz played 35 tournaments this year. The only other player in the top 100 on the world rankings that played as many events is Im Sung-Jae of South Korea.
Thanks to his phenomenal season, Jazz has qualified for three of the four majors in 2020 - the Masters, PGA Championship and British Open.
He has also earned berths at two World Golf Championships events, the WGC-Mexico and WGC-FedEx St Jude Invitational.
It was at the 2019 PGA Championship in May where Jazz rose to global prominence.
He was a popular player at Bethpage Balck and was joint second after three rounds.
Jazz said he enjoyed the noisy New York crowds.
"It's the first time I have had a crowd like this, shouting my name. I didn't know how to react at first," he said.
Unfortunately, he cracked under pressure in the final round to finish tied for 14th.
It was still the best finish by a Thai in the PGA Championship and second best in a major behind Thongchai Jaidee's 13th place at the 2009 British Open.
While he was disappointed, he chose to look on the bright side.
"What happened is good because I'm only 23, playing in my second major," he said.
"If I finished too good, I could be lazy, I could be thinking I was a big shot. That humbled me. Now I know I need to work a lot harder."
He did work harder and his efforts paid off handsomely.
He won the Korea Open in June and ended his long season with triumphs in Jakarta and Pattaya.
Jazz was crowned the Asian Tour Order of Merit champion of 2019 even before he teed off in Jakarta.
"It is indeed a very good year. I had four wins this year and I ended the year the way I started," Jazz said after the Thailand Masters.
The Hua Hin native won the season-opening Singapore Open, co-sanctioned by the Japan Golf Tour, in January when he defeated a field which featured Sergio Garcia and Paul Casey.
Following the win, he cracked into the world's top 100 and aimed to be in the top 50 by the end of the season.
He moved into the top 50 with his Indonesian Masters win and then targeted a top 40 spot ahead of the Thailand Masters.
He met the goal with his triumph at the Phoenix Gold Golf and Country Club.
He is now the highest-ranked Thai male golfer at No.40, well ahead of No.106 Kiradech Aphibarnrat.
Kiradech, Thailand's first and still only PGA Tour member, is one of Jazz's golfing idols.
One of Jazz's desires is to join Kiradech on the PGA Tour. "That's my goal... hopefully in the near future," he said.
Jazz has now won six Asian Tour titles and he does not intend to slow down.
"I want to get even better and do not want to rest on my laurels. I want to compete in more majors," he said.
Having secured berths at the 2020 Masters, PGA Championship and British Open, there is still time for Jazz to earn a place at the US Open.