On Monday morning, former state Rep. Marty Walsh will be sworn in as Boston’s first new mayor in two decades. For him, the transition from the State House to City Hall will likely be a huge shift. He will go from being one person in the middle of the hundreds who populate the three branches of state government to the person with his hands on all the levers of power in the city of Boston.

As he familiarizes himself with City Hall from the point of view of the top job, Boston will be familiarizing itself with its new leader. While a member of the Boston delegation on Beacon Hill for 16 years, Walsh is unknown to large swaths of the city.

-- Martin Joseph Walsh is 46 years old, the son of Irish immigrants who met at a dance hall on Dudley Street. He lives on Tuttle Street, a nine-minute walk from the three-decker he grew up in on Taft Street.

-- While he can be impulsive at times, he also likes to take his time on matters, like picking a chief of staff. “Politics is funny,” he told the Reporter in 2008. “It’s like recovery: a day at a time.” He often speaks off-the-cuff in his formal remarks. A man with an average-guy persona, he will have to get used to having an entourage as mayor.

-- Walsh was elected to the State House in 1997, beating out future state Attorney General Martha Coakley and Jim Hunt III, Mayor Menino’s former environmental chief, among others. He came in as a “hotheaded guy from Dorchester” during a prosperous time for the neighborhood, as then-Speaker Thomas Finneran, a Mattapan Democrat, funneled money towards their adjoining districts. Walsh picked up a second job as head of the Building and Construction Trades Council in 2010, before stepping down to run for mayor in April. He was a founding board member of Dorchester’s Neighborhood House Public Charter School.

-- He cites the late Congressman Joe Moakley (D-South Boston) as a political role model. “He was beloved by his constituents, and dedicated almost his whole life to public service,” Walsh said in a Reporter questionnaire sent out to the mayoral candidates. “He was a great collaborator, and one of the most effective legislators of our lifetime. He was a champion in areas important to me, like housing, health care, elderly and veterans services. He was a man of integrity, intelligence and compassion.”

-- He has an inner circle that is rarely set in stone, and it includes State House colleagues and childhood friends. On the night of his preliminary win, he couldn’t sleep, so he called several people to thank them for their support. Among them: Joyce Linehan, a communications consultant who lives in Lower Mills and worked on Gov. Deval Patrick’s campaigns and Elizabeth Warren’s Senate run; Joe Rull, a former aide to Mayor Menino and field director for US Rep. Stephen Lynch; and the North End’s Stephen Passacantilli, who had worked for East Boston Councillor Sal LaMattina. Walsh is close with state Rep. Eugene O’Flaherty, a conservative Democrat from Chelsea and head of the House side of the Judiciary Committee. Walsh’s longtime girlfriend and best friend is Lorrie Higgins, a Judiciary Committee aide.

-- Walsh is one of three Martin Walshes in Boston politics. The others include his cousin, Martin F. Walsh, a labor leader who was sometimes referred to as “Big Marty” within the campaign, and Martin Walsh, the former aide to the late U.S. Sen. Ted Kennedy. The former Kennedy aide received a congratulatory call on Election Night from Vice President Joe Biden that was meant for the mayor-elect.