Waltham Mayor Jeannette McCarthy will not seek re-election after her fourth term ends in 2020, she announced in a July 14 letter to the City Council.

"As a courtesy, I wanted you to hear it from me, that this will be my last term as Mayor," McCarthy wrote in the letter, which was verified for the News Tribune by Assistant City Clerk Joe Vizard.

McCarthy has served 13 1/2 years in office, having first been elected in 2003. Prior to serving as mayor, she spent two years as a councilor-at-large and seven years as a city solicitor, according to her biography on the city’s website. She also served six years on the Waltham School Committee before becoming mayor.

Though she gave no explanation for her decision not to seek another term, McCarthy wrote in the letter that she will "continue to work diligently to do the people’s business and finish a multitude of projects" throughout the remainder of her term.

McCarthy had said in 2003, when she was running for the first time, that she would not run for more than two terms. She later said that she would not seek a fourth term in 2015, though she went on to defeat State Rep. Tom Stanley with 55.1 percent of the mayoral vote in that election.

McCarthy was unavailable for immediate comment.