"I have enjoyed serving the last 8 years as Mayor/CEO of the Unified Government of Wyandotte County/Kansas City, Kansas," he wrote in the letter. "Together, we have accomplished so much and have moved this community forward."
He said he made the decision while spending time with his family over the holidays.
"I wanted to leave (Kansas City, Kan.) better and stronger than before, and I think I've done that," Reardon said. "Is there room for improvement? Is there more to be done? Yes."
Reardon served two terms as mayor.
He said Wednesday that he plans to stay engaged with the community, but would not say whether or not he will get into politics again.
"For now, I'm going to go home and be with my family," he said.
According to his official bio, he is a native of Kansas City, Kan., and graduated from Bishop Ward High School.
His accomplishments as mayor include the arrival of Sporting KC in Kansas City, Kan., and Google Fiber selecting the city as its first choice to build and operate the high-speed system.
In 2012, the Kansas Mayors Association honored Reardon as Mayor of the Year.