Donnelly, whose district includes Arlington, Billerica, Burlington, Lexington, and Woburn, was elected to the Senate in 2008. He had been serving as assistant majority leader, according to the Legislature’s website.

State Senator Kenneth Donnelly, an Arlington Democrat and 37-year veteran of the Lexington Fire Department, has died from complications of brain cancer, according to a statement from his office. He was 66.

In August, after not feeling well during votes on Beacon Hill, Donnelly sought medical help at a walk-in clinic. He was then taken to Massachusetts General Hospital, where doctors operated on a cancerous brain tumor.

Cindy Friedman, Donnelly’s chief of staff, said in a statement Sunday night that the senator “gallantly battled” the tumor for eight months.


Senator Eileen Donoghue, who sat next to Donnelly in the Senate chamber, described her colleague as a “fighter from the get-go.” Donoghue said he had returned to the State House after his surgery and had attended many of this year’s formal caucuses.

“When I think about Ken, I just think about how he always stood up for the working man and woman,” she said. “It’s a void that will be almost impossible to fill.”

Governor Charlie Baker and Boston Mayor Martin J. Walsh said Sunday night that Donnelly will be missed. Baker tweeted that he and the senator “go back 20 years.” Walsh tweeted that Donnelly was a “true champion for working families & advocate for equality.”

Donnelly was a “fighter for those without a voice,” Friedman said, and his work focused on a variety of issues, including the strength of unions, protection for homeless families, and access to mental health care.

“Through all these diverse efforts,” Friedman wrote, “Senator Donnelly never sought the accumulation of personal credit; but rather he was dedicated to the causes he believed in and the people he represented, and he brought his tremendous energy, courage, and passion to changing many lives for the better.”


Donnelly wrote an editorial, published in Friday’s Globe, detailing three proposed changes that lawmakers could make to improve the state’s mental health care systems.

Public safety was an important priority for Donnelly because of his experience as a firefighter, Donoghue said. He came from a long line of public safety officials in his family, including police officers and firefighters, she said.

Many of Donnelly’s colleagues said he was friendly to everyone — humorous and compassionate, they said, but passionate about his missions.

“You felt his presence if you were around him,” said Representative David Nangle.

State Senator Jamie Eldridge said Donnelly fought endlessly for what he believed was right, even when he didn’t have allies.

“The memory a lot of us have of Ken within the State House is that he was someone who really wore his heart on his sleeve,” he said. “He wasn’t afraid to speak very passionately within the caucus.”

Yet Donnelly was reasonable, said former Senator Steven Panagiotakos. He understood that negotiation was often necessary to make progress.

“He underscored the word good, and that’s what he was,” he said.

Donnelly has three children and five grandchildren, Friedman said. He had been married to his wife, Judy, for 43 years.

He also loved to cook for family and friends, Donoghue said, and Donnelly had a kitchen like that of a professional chef.

“I’d always ask him, ‘What are you making for Easter, or what are you making for Thanksgiving, or what are you making for Christmas?’ He had his menu all set, and he didn’t care how many people showed up,” she said. “He was going to be able to feed everybody.”


Felicia Gans can be reached at felicia.gans@globe.com. Follow her on Twitter @FeliciaGans.