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Dave Sweeney, who has been Mayor Martin J. Walsh’s behind-the-scenes chief of staff for the last 2½ years, is planning to leave for the private sector by the end of February, he said.

Sweeney, the city’s one-time budget chief, accepted a job as president and chief executive of MASCO, the Medical, Academic, and Scientific Community Organization that manages operations for the institutions that make up Boston’s 210-acre Longwood medical and academic campus.

“It became clear that this is a really good fit with the skills I have and what I want to do in the long term,” Sweeney said in an interview, adding he long envisioned working in the nonprofit sector after leaving City Hall. “Those leadership opportunities don’t come up every day, and so when it did, I thought it was a good opportunity to pounce on it.”

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He leaves at a critical time for the Walsh administration, which the mayor has described as being in the “implementation stage” of his policy visions, halfway through his second term, and as city councilors and community advocates have been pushing the mayor to go harder to the left on issues such as transportation and housing.

The administration has been embroiled in recent controversies, as well, such as a recent bribery scandal, and the extortion convictions of two of Walsh’s top aides — issues not of Sweeney’s making, but that nonetheless demanded an administrative response.

Walsh lavished praise on his top aide.

"Dave Sweeney has been a constant presence in my administration for the past six years, where . . . he’s focused successfully on how to best deliver the services our residents want and need to live a quality life in Boston,” Walsh said in a statement, praising the city’s financial stability that has allowed for “historic investments that will move Boston and its neighborhoods forward.”

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“Dave's dedication to Boston's long-term growth and stability is unwavering and I want to congratulate him on his new position,” Walsh said.

The mayor said he will likely appoint an interim replacement and conduct a comprehensive search for a new chief of staff, saying he was confident in “a seamless transition in continuing this work.”

Sweeney, 37, who lives in Dorchester with his wife and two young children, was seen as an under-the-radar operative compared to his predecessor, Daniel Koh, who left in the summer of 2017 to unsuccessfully run for Congress.

Koh was Walsh’s first chief of staff and came in with a big-picture agenda for the new administration. But the aim-for-the-stars mentality came with public blunders, such as the 2024 Olympics debacle. The administration had also mishandled a controversial proposal for an IndyCar race car event in South Boston. All the while, the first of two federal criminal investigations into City Hall activities — related to the extortion of the Boston Calling music festival — rocked City Hall.

Sweeney’s assignment was to steady the ship as Walsh cruised through his first reelection campaign and began to implement his priorities on housing and development.

He had previously served as Walsh’s head of administration and finance, overseeing the city’s $3 billion-plus budget. Before joining the administration, he worked for the Lottery Commission and as budget director of the state House of Representatives Committee on Ways and Means, and as an adviser to House Speaker Robert A. DeLeo.

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As chief of staff for Walsh, Sweeney was a fixer and a top policy adviser to the mayor. He corralled all of the city’s cabinet-level chiefs — representing housing, environmental, health and human services, and transportation departments, as well as the public safety sector — to carry out the mayor’s agenda.

The city has been coasting on a booming economy, as well, boasting six consecutive AAA bond ratings, a perfect score.

“I don’t think there is anyone who would say the city’s finances aren’t in better shape than they were in 2014,” he said. “I think my job was to keep the city moving.”

Sweeney would be the most high-profile official to leave the administration since former schools superintendent Tommy Chang and police commissioner William Evans departed at around the same time nearly two years ago. It’s not unheard of for a big-city chief of staff to serve for only a few years. Walsh’s predecessor, Thomas M. Menino, had multiple chiefs of staff during his 20 years in office.

At MASCO, Sweeney will oversee a 47-year-old organization that represents institutions with a combined 57,000 employees and 29,000 students, and that see more than 2.3 million patients annually.

“Dave offers the right mix of public-policy expertise, capacity to build partnerships with both private and public institutions, and operational skills to lead MASCO going forward,” said Dick Argys, chair of the organization’s board of directors.

He said the organization tapped Sweeney after a “comprehensive,” regional search.

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Sweeney said in a statement that he was looking forward to joining an organization that, he said, serves as an economic engine for the city, as a source of employment and center for health care and higher education.

“In many ways, the [Longwood Medical Area] reflects the best of Boston’s brand as an innovative global leader,” he said.


Milton J. Valencia can be reached at milton.valencia@globe.com. Follow him on Twitter @miltonvalencia.