BOSTON - Boston has a new mayor. Kim Janey, who took office on Monday, became the first African-American and first woman to lead the city.

Mayor Marty Walsh resigned Monday evening to become President Biden’s labor secretary. As president of the Boston City Council, Ms. Janey stepped into the role of acting mayor and had a ceremonial swearing-in Wednesday.

Mr. Walsh, the latest in a long line of largely Irish-American Boston mayors stretching back the better part of a century — with one notable Italian- American exception — said he welcomed the change.

“History will be made tonight,” Mr. Walsh said earlier Monday evening. “We’re an extremely diverse city from different backgrounds and different nationalities and different skin colors. I think it’s a good thing for our city. I think it’s a great thing for our city.”

Mayor Janey took to Twitter to wish Mr. Walsh well following his confirmation by the U.S. Senate.

“Congratulations on your confirmation, Secretary Walsh. You are a proud son of Dorchester who will bring our city with you,” she tweeted. “The working people of America will benefit greatly from your passion.

“Now, we look ahead to a new day—a new chapter—in Boston’s history,” Mayor Janey, a fellow Democrat, added.

Mr. Walsh said for the past two months he has had regular meetings and conversations with Mayor Janey. The two have also held extensive planning sessions, he said.

“Together the council president and myself and our teams have worked diligently to ensure a smooth transition,” he said.

By any typical political stopwatch, Mayor Janey’s rise has been lightning quick. She was first sworn in as a city councilor three years ago.

Although Mayor Janey, 55, is holding the office only on an interim basis, she’s widely seen as hailing a new chapter in Boston’s political history.

Those actively seeking the office include three women of color — current city councilors Michelle Wu, Andrea Campbell and Annissa Essaibi George. John Barros, who is of Cape Verdean descent, and state Rep. Jon Santiago also are running. Mr. Barros served as chief of economic development under Mr. Walsh.

Mayor Janey has a long history of activism in Boston, with deep roots in Roxbury, the heart of the city’s Black community.

Her grandfather, Daniel Benjamin Janey, was a member of Twelfth Baptist Church where Dr. Martin Luther King Jr. worshipped while attending Boston University. Her father was one of only eight Black students to graduate from the city’s prestigious Boston Latin School in 1964.

While spending time in her great-grandmother’s home in the city’s South End neighborhood, Mayor Janey also was exposed to the city’s political culture as she watched a neighbor — Black community activist and former state Rep. Mel King — launch a bid for mayor in 1983, losing to Ray Flynn, an Irish-American city councilor.

During the second phase of Boston’s tumultuous school desegregation era, Mayor Janey would recall the rocks and racial slurs she said were hurled at her as an 11-year-old riding the bus to the largely white neighborhood of Charlestown. She would later take part in a program that allowed her to attend school outside the city.

Mayor Janey began her career in advocacy with Massachusetts Advocates for Children, pushing for policy changes she said were aimed at ensuring equity and excellence for public school students in Boston.

In 2017, she won a 13-candidate race and became the first woman to represent her district, which includes most of Roxbury, parts of the South End, Dorchester and Fenway areas of the city.

Although she hasn’t said if she’ll run for mayor in the fall, there is precedent for an interim mayor using the temporary post as a stepping stone to winning the seat outright.

When former Mayor Raymond Flynn stepped down to become President Bill Clinton’s ambassador to the Vatican, then-city council president Thomas Menino stepped in as interim mayor in July 1993, won the mayoral election later that year, and served in the office longer than anyone in the city’s history.