The Royals rotation took a hit earlier this offseason when right-hander Ervin Santana and left-hander Bruce Chen opted for free agency.
The Mendoza news came as he and his wife, Monica, were celebrating their third wedding anniversary on a trip to Europe.
"I'm very thankful to the Royals for all the opportunity they gave me and for being part of this great team and beautiful city," Mendoza said via email. "The team year by year is getting better and better because they have put together a great group of guys with awesome talent."
Mendoza, who turned 30 on Oct. 31, began last season in the rotation after beating out Chen for the fifth spot during Spring Training.
The product of Veracruz was on a roll, coming off the Caribbean Series championship with Mexico. He was named the tournament's Most Valuable Player after pitching 13 1/3 scoreless innings in two starts. He also pitched for Mexico in the World Baseball Classic.
Mendoza started for the Royals through July 7 and then was replaced by Chen, who came out of the bullpen while Mendoza was reassigned to relief. He was doubtful that he would have gotten strong consideration for the rotation next year.
"About being in the rotation in 2014, I don't think so," Mendoza said in his email. "They didn't see me as a starter -- maybe in the bullpen but that's something I can handle. They make the decisions of who is going to make the team and in what role."
After his midseason switch to the bullpen, Mendoza started hearing of interest from Japan. Later came a substantial offer to be a starting pitcher.
"Japan offered me a good contract that in the States nobody was going to give me," Mendoza wrote. "I'm totally convinced that Japan is a great league with a great level of the game. I'll have to work hard and be at the same high level I was for the big leagues."
In his 15 starts for the Royals, Mendoza had a 2-5 record and a 5.01 ERA. After being moved to the bullpen, he was used only six more times after the All-Star break. Overall, he worked in 22 games, going 2-6 with a 5.36 ERA.
One of his best outings came on June 9 against the Astros when he turned in seven shutout innings, giving up just four hits and one walk. Relievers Aaron Crow and Greg Holland finished up his 2-0 victory. For Mendoza, it was his first career start of more than five scoreless innings.
That game was at Kauffman Stadium, where he struggled most of the time with a 7.08 ERA and where opponents hit .332 against him. He fared better on the road, where he had both of his victories, a 3.55 ERA and held opponents to a .240 average.
Obtained for cash from Texas, Mendoza was with Kansas City for parts of 2010 and 2011 and all of the last two seasons. In 2011, his 2.18 ERA for Triple-A Omaha led the Pacific Coast League. His total record for the Royals was 12-17 with a 4.72 ERA in 58 games (42 starts).
During his stay in Kansas City, Mendoza provided a veteran voice, especially for Latino players, and participated in several community and charitable events.
Including his time with the Rangers, Mendoza has a Major League record mark of 16-25 and a 5.39 ERA in 90 games.