Jack Ethredge, who has led Thornton for 32 years as city manager and utilities director, has announced he will retire March 20.
Since taking office on Oct. 22, 1984, Ethredge has seen Thornton grow to the sixth-largest community in the state.
Ethredge notified City Council Jan. 10 of his intention to retire.
"The evolution of the city over these years has truly been transformative for the community and the city organization," said Ethredge in his resignation letter to Thornton City Council. "Jean and I are living proof that Thornton is a great place to live, work and raise a family. I came to Thornton for a job and found a home."
Mayor Heidi Williams credited Ethredge's leadership and efforts with much of the city's successes.
"He came to this community at a critical time when it needed stability and solid direction - Jack provided that for the residents and businesses within Thornton," Williams said. "He has worked with many city councils since being hired and has found a way to effectively bring these people together to provide for the present and the future for this great community."
The council said it will begin the search for a new city manager in the next few weeks.
Ethredge praised Thornton and its residents for their commitment to always working to better the community.
"No community, including this one, is perfect but the spirit of this community and the many and ongoing efforts - be it through public, nonprofit, faith-based or private sector - to enhance our community have been and are remarkable," he said in his letter. "It is especially remarkable and somewhat unusual to have had the opportunity to see and appreciate all these efforts over these years by those engaged in the development of this community."