Boston's former top prosecutor was sworn in Monday as the U.S. Attorney for Massachusetts, becoming the first Black woman to serve as the top federal prosecutor in the state.

Rollins took the oath of office in a private ceremony in the Boston federal courthouse after officially stepping down as district attorney for Suffolk County, which covers Boston and three suburbs. A formal investiture is expected to be held later.

The 50-year-old Boston Democrat said in a statement that Monday's ceremony was a “proud and humbling moment” made all the more special because her parents were in attendance and it came on her father's birthday.

“Their support has been unwavering, and I owe so much of this moment to them,” she said.

Rollins takes over an office comprised of more than 250 federal prosecutors and staff across three locations in Boston, Worcester and Springfield.

Rollins was narrowly confirmed by the U.S. Senate last month, with Vice President Kamala Harris casting the deciding vote.

She has said she received violent and racist threats as a result of her nomination and that the U.S. Marshals Service still denied her request for a security detail, deeming she was at low risk.

Rollins earned her law degree from Northeastern University in Boston and previously served as legal counsel for a number of public agencies, including the Massachusetts Port Authority, state Department of Transportation and the Massachusetts Bay Transportation Authority. She also was a prosecutor in the office she now runs from 2007 to 2011.

Rollins was elected district attorney in 2018, becoming the first woman of color to serve in such a capacity in the state. During her tenure, she pushed for progressive criminal justice reforms, including pledging not to prosecute certain low-level crimes to better focus on more serious crimes like homicide.

Gov. Charlie Baker has appointed Kevin Hayden, chair of the state’s sex offender registry board, to serve out the remainder of Rollins’ district attorney term, which ends later this year.

Hayden, who also took over Monday, said one of his priorities will be to reduce the number of illegal guns in the county.

“The harm that guns cause is incalculable and we must do everything in our power, use every tool at our disposal, to reduce that harm," he said in a statement.