And crises abound, particularly at the MBTA, which remains subject to unusual federal oversight and plagued by slipshod service.

When Healey appointed Fiandaca in December, the governor-elect hailed her as a visionary transportation leader with “decades of experience managing large transportation departments and prioritizing safety, reliability and accessibility.”

Neither Fiandaca nor the administration was offering explanations Monday about what triggered the departure.

Healey aides said the governor was not available for an interview Monday. Healey traveled on Sunday to Aspen, Colo., for a two-day Democratic Governors Association retreat.

Asked whether Healey had asked for Fiandaca’s resignation, Karissa Hand, a Healey spokesperson, said Fiandaca “made the decision to step down.”

But Hand did not directly address other questions, including why Fiandaca is leaving. She did not say whether top Fiandaca hires will remain on at MassDOT, nor whether the governor has a timeline for naming a permanent replacement.

Undersecretary of Transportation Monica Tibbits-Nutt, a former member of the Massachusetts Department of Transportation board of directors and MBTA Fiscal and Management Control Board, will become acting secretary of transportation. Fiandaca will remain in an advisory role through the end of the year, according to Healey’s office.

It was not immediately clear whether Fiandaca will continue to earn her $181,722-a-year salary once she moves into her advisory role and someone else takes the reins.

In a statement, Healey said she and Lieutenant Governor Kim Driscoll thanked Fiandaca and “wish her well in all of her future endeavors.”

“She hit the ground running and has delivered on many of our key transportation priorities,” Healey said.

Before taking over as Healey’s transportation secretary in January, Fiandaca worked as an assistant city manager overseeing mobility for the city of Austin, Texas. Before that, she was the transportation commissioner for the city of Boston under former mayor Martin J. Walsh. Earlier, she was director of the city’s Office of the Parking Clerk.

The secretary’s announced departure caught many off-guard in political and transportation advocacy circles.

“I was a little surprised,” said Leominster Mayor Dean Mazzarella, a MassDOT board member since 2015. Mazzarella said he received a phone call late Monday morning about the announcement.

“[Fiandaca] was very confident, just a nice person, and extremely capable,” he said. “I didn’t see any conflict anywhere.”

Ultimately, her tenure will be among the shortest for a position that, until recently, had a history of high turnover. Healey’s predecessor, Charlie Baker, had just two transportation secretaries over his eight-year tenure, with Stephanie Pollack’s six years in the role standing as one of the longest stints in decades.

Before that, former governor Deval Patrick cycled through several secretaries over eight years, with Richard Davey, now the president of New York City Transit, following Jeff Mullan, James Aloisi, and Bernard Cohen. Aloisi served just 10 months as secretary in 2009.

Several people voiced confidence in Tibbits-Nutt’s ability to step into the role, given her several years as a member of the former iteration of the MBTA’s board. She didn’t respond to a request for comment Monday.

“Undersecretary Tibbits-Nutt has a proven record of being dedicated to transportation justice,” said a statement from advocacy group TransitMatters. “Her thorough understanding of the transportation system makes her the perfect person to shepherd our transportation system through this rebuilding period in a way that regains trust and ridership.”

But the quick turnover represented the first notable fissure in Healey’s still-fledging leadership team, never mind in such a public-facing role.

State Senator Brendan Crighton, a Lynn Democrat who co-chairs the Legislature’s transportation committee, said he “never got the sense” Fiandaca and Healey weren’t on the same page.

“We work with a wide range of folks over there. It’d be hard for me to pinpoint any of that stuff,” he said. “For all my time in the governor’s first year, it’s been great. There are some big challenges. . . . [But] Monica’s been there, too. They’re not going to miss a beat.”

State Representative William Straus, the longtime House chair of the transportation committee, said Fiandaca “can feel proud of” of several developments in her short stint, including the rollout of driver’s licenses for undocumented immigrants.

But, he said, Healey has populated the MBTA and MassDOT with a broad enough roster of appointees that it shouldn’t affect the direction of projects or priorities.

”I don’t think there’s any detriment to the public interest here,” the Mattapoisett Democrat said. “Monica comes as well-prepared as any secretary that I’ve worked with over the years to step into the position.”

Fiandaca’s tenure was marked by successes and what critics called missteps.

The Sumner Tunnel closure this summer, requiring coordination between MassDOT, the city of Boston, and Massport, is seen as having been as painless as possible, with travel delays far less severe than anticipated.

But already bad conditions for riders of the MBTA have continued to deteriorate under Fiandaca’s leadership, with a large uptick in the percentage of subway tracks requiring speed restrictions because of faulty infrastructure and several serious safety incidents, including crumbling station ceilings that injured at least one rider.

Also under Fiandaca, in May, MBTA and state officials entered into a $900,000 no-bid consulting contract with Teneo Strategy LLC, under which Bill Bratton, the former Boston police commissioner and Fiandaca’s ex-brother-in-law, was slated to advise Fiandaca and the MBTA as the agency worked to respond to a host of federal safety directives. The move — Bratton told the Globe that Fiandaca first approached him — came less than a year after state investigators criticized the T for entering into no-bid contracts.

MBTA general manager Phillip Eng said Fiandaca has helped the agency lay the foundation of “safe, reliable, and more frequent service that the public expects and deserves.”

“I greatly appreciate the trust that Secretary Fiandaca has shown in me by allowing me the opportunity to lead the MBTA during such a transformative period in our history,” he said in a statement.

In a statement, Fiandaca celebrated her short stint at MassDOT.

“I wish the Healey-Driscoll Administration continued success,” she said.

In a note to MassDOT employees Monday obtained by the Globe, Fiandaca touted accomplishments such as a new labor agreement with the MBTA’s largest union and onboarding Patrick Lavin as chief safety officer at MassDOT.

“While I have enjoyed the challenges and responsibilities at MassDOT and the MBTA since January, I have come to a decision to leave my position on September 11, which will allow for a smooth transition,” Fiandaca wrote.

Transit advocates say while the departure comes at a crucial time, the administration has already secured an MBTA general manager and a safety chief who have begun doing important work on the T.

Having Tibbits-Nutt step into the role will make the transition effectively seamless, said Stacy Thompson, executive director of LivableStreets Alliance, a public transportation advocacy group. “We couldn’t be in a better position,” Thompson said.

Thompson added that in searching for a new secretary, the Healey administration should look to Tibbits-Nutt or another expert “who wants to stick with this for the long haul” and has an interest “in the boring work.”

“This job,” Thompson said, “is less about ribbon cuttings and more about operations.”

Samantha J. Gross of the Globe staff contributed to this report.