When Mayor Thomas Menino said he would not attempt to add another four-year term to his 20-year tenure, there was little doubt that Dorchester state Rep. Marty Walsh would take a crack at the seat opening up on City Hall’s fifth floor.

On Saturday, before a crowd of over 1,000 supporters at the Strand Theatre, Walsh strode onto the stage and looked out into the audience, his voice straining with emotion as he recounted his life story.

Mary Walsh, his mother, stood off to his left. “He’s always talked about running for mayor,” she said, after the confetti cannons belched out the red, white, and blue ticker tape. “He always talked about politics.”

Mary emigrated from Ireland in the 1950s, marrying the late John Walsh, a union laborer and fellow immigrant, and moving to St. Margaret’s Parish.

In the neighborhood, Walsh quickly became friends in grammar school with Frank Baker. The District 3 city councilor, who is up for reelection this fall, took to the Strand stage on Saturday and pledged to “eat, sleep, breathe Marty Walsh. I’m with Martin J. all the way,” he yelled.

State Rep. Liz Malia, a Jamaica Plain Democrat, also voiced her support for Walsh, whom she got to know at the State House as they worked together on substance abuse issues. “He’s a mediator, he’s a bridge builder,” she told the crowd.”

They were joined on the stage by state Rep. Eugene O’Flaherty, a Chelsea Democrat. Margaret Lynch, the wife of Congressman Stephen Lynch, was in the audience.

In a sit-down with the Reporter at the Haley House two weeks after Menino’s announcement, Walsh cited his 16 years as a state representative for Dorchester working on such issues as turning a Hallet Street dump into Pope John Paul II Park, improving Dorchester’s beaches, and ensuring that the struggling Carney Hospital stayed open. “I have a lot to offer the city,” Walsh said. “I love the city.”

The diverse district he represents is a “microcosm” of the city, he said, including Codman Square, Fields Corner, Pope’s Hill and Savin Hill.

Walsh said he has helped craft budgets. The city councillors who are running for mayor are a part of the city’s budget process, he pointed out, but they perhaps don’t have the “same expertise.”

Walsh has resigned from his position as a top union official with the building trades, he said, so it won’t be a “distraction. I’m in this race to win.”

After Walsh’s speech, a 90-year-old woman who had been sitting in the front row scrambled to the stage and started to shovel the confetti into her handbag. “Marty for Mayor” stickers were all over her cane. She said she planned to bring the handbag to the fifth floor after the election and show it to Mayor Walsh. “This is from the Strand Theatre,” she said she plans to say.