WALTHAM — Longtime Mayor Jeannette A. McCarthy will not seek reelection when her term ends in 2020, she announced in a letter to city councilors and employees.

McCarthy, who was first elected to office in 2003, sent a letter July 14 saying she would not seek a fifth term, City Council President Diane LeBlanc said.

“The mayor has served the city well,’’ LeBlanc said. “She has certainly done her time.’’

McCarthy, 64, declined to comment at Waltham City Hall on Monday night.

In her letter to employees, she did not give a reason for not seeking another term as mayor of her hometown.

“As a courtesy, I wanted you to hear it from me, that this will be my last term as mayor,’’ McCarthy wrote. “Over the next 2 ½ years, I will continue to work diligently to do the people’s business and finish a multitude of projects.’’

McCarthy also thanked employees for their “hard work, dedication, and help’’ with the projects and tasks of city business during her tenure as mayor.

McCarthy, a graduate of Waltham’s public schools, served on the School Committee from 1986 to 1991 and was an at-large city councilor from 2002 to 2004.

According to a biography on the city’s website, she is a graduate of Boston College, Suffolk University Law School, and Northeastern University.

McCarthy also served as city solicitor and had a private law practice. She has been a volunteer with several organizations, including the American Cancer Society.

In her first run for mayor in 2003, she defeated Thomas Stanley with 70 percent of the vote.

In the fall of 2015, McCarthy won a fourth term and defeated Stanley, a state legislator and a City Council member, with 6,074 votes to Stanley’s 4,948.

John Hilliard can be reached at john.hilliard@globe.com.