Portland City Manager Mark Rees announced Monday that he will resign early next month after more than three years on the job, creating another top-level administrative vacancy at a time of robust development and positive national attention for Maine’s largest city.

Rees said in a letter sent Monday to city councilors that he will step down Sept. 3 “in order to pursue other opportunities, both professional and personal.”

In his letter and a brief interview, the city’s top administrator said he was proud of the three balanced budgets his administration put together, implementation of the city’s first five-year capital improvement plan and hires he made for key city positions. He declined to discuss the reasons for his resignation.

“I’ve enjoyed Portland immensely,” Rees said. “It is a great city and it’s been a pleasure and privilege to work here.”

Rees’ resignation comes four months after the City Council voted to extend his contract by one year – a shorter time frame than the three-year pact he was given when hired – which some interpreted as a possible sign of friction between him and the city’s elected officials. The search for a new city manager will take place as the city tries to fill a string of other high-level vacancies, including the directors of finance, human resources, and health and human services.

Rees’ tenure included several public spats with city councilors – including complaints that he did not notify them quickly enough after the city’s firefighting boat ran aground in 2011 – and disagreements with Mayor Michael Brennan over their balance of power in city government.

But Monday, councilors largely praised Rees’ job performance.

Brennan credited Rees with management changes that improved efficiency within city government, and praised him for numerous strong hires. Brennan said Rees told councilors about a week ago that he planned to resign.

Brennan said the City Council did not ask for Rees’ resignation and insisted there was “no cause and effect” between the length of his contract and Rees’ decision.

“I had a good working relationship with Mark and I enjoyed working with Mark,” said Brennan.

Rees was hired in May 2011 after serving more than a decade as city manager of North Andover, Mass. He was Portland’s first city manager with a formal employment contract. Before recent changes to the city’s charter – changes that also made the mayor’s job an elected position – the council set the city manager’s salary and benefits annually. Rees was scheduled to earn just under $146,600 for the next year, plus a $450 monthly automobile allowance.

Rees said in April that he hoped to stay in Portland beyond the June 2015 end date of his contract extension.

“We set some common goals we wanted to achieve and I think they would like to see progress,” Rees said at the time.

City Councilor Ed Suslovic said Rees’ resignation “didn’t completely surprise me.” He noted that Rees had fulfilled his original three-year employment contract and that the average tenure for heads of municipalities and universities is three to five years.

“He has certainly fulfilled his commitment to the city,” Suslovic said. “He’s certainly leaving the city in a good place.”

Suslovic said Rees’ top accomplishment was refining the process for establishing a five-year Capital Improvement Plan for large-scale public projects that require borrowing. Suslovic also mentioned Rees’ hires of a fire chief, police chief and planning director.

Suslovic declined to talk about whether he thought Rees had shortcomings, or to discuss a recent performance evaluation.

Portland continues to receive positive national attention. Suslovic said the departure of Rees is a “potential opportunity.”

“I think, frankly, this is a good time for Portland to put out the opportunity for someone to move to a city that’s on everyone’s top 10 list,” Suslovic said. “So my hope would be that we would attract some of the best and brightest from around the country, because Portland does have an outstanding reputation as a city on the move.”

Rees’ tenure coincided with a period of significant growth. Portland’s total assessed property value jumped $87 million during the past year – the largest increase since before the Great Recession – thanks to several large hotel projects and various real estate developments. The city’s thriving culinary and arts scene are also drawing national attention.

But Portland also faces numerous challenges, including a growing homeless population, shrinking state financial support and recent tensions with the LePage administration over whether to provide General Assistance for undocumented immigrants. The city has also been taken to court over ordinances concerning panhandling on street medians, protests outside an abortion clinic and the attempted sale of part of Congress Square Plaza.

During Rees’ tenure, the city’s undedicated fund balance grew from 9.5 percent of the General Fund to 12.2 percent. City officials also credited Rees with reorganizing the Fire Department and streamlining the development permitting process.

“He established the city’s first five-year capital improvement program, as well as presenting us with three balanced operating budgets,” Councilor Nicholas Mavodones, chairman of the Finance Committee, said in a written statement. “I wish Mark well as he pursues other opportunities.”

Other members of the City Council were not available or did not respond to requests for comment Monday evening.

Carol Schiller, president of the University Neighborhood Organization, which advocates for the area surrounding the University of Southern Maine campus, said she was shocked and saddened to hear that Rees is leaving. She praised Rees for “welcoming” community associations to interact with him and other city staff, pointing out that he often had department heads attend the Neighborhood Advisory Committee meetings with leaders of the various associations.

“It is Portland’s loss that he is leaving,” Schiller said.

The City Council is expected to appoint an interim city manager Sept. 3. The city said in a media release that the search for a new city manager will begin immediately.

Staff Writer Randy Billings contributed to this report.