Then, taking the baton, she showed what a dynamic presence she is on the podium.
"To become a conductor, whether you're a man or a woman, it's a tremendous challenge," Alsop, the conductor of the Baltimore Symphony Orchestra, told CBS News correspondent Anthony Mason.
The daughter of musicians, Alsop took up the violin at age 5. But at 9, she saw Leonard Bernstein and knew that she wanted to be a conductor.
"And then I particularly remember telling my violin teacher and hearing these words that I never thought I'd hear in my life: 'Well, girls can't do that,'" she said.
Her father immediately bought her a box of batons. Later, at the prestigious Tanglewood conducting program, Alsop would meet her mentor.
"And Bernstein walked in," she said. "And he looked at the sea of people and he said, 'well, where's this Marin?' You know, and I thought 'oh I have died and gone to heaven!'"
She led orchestras in Colorado and Bournemouth, England. But she nearly turned down the Baltimore job.
When Alsop was hired, the Baltimore Symphony was struggling to fill seats. The orchestra was in debt. And it hadn't made a recording in nearly a decade.
But that's all changeing. Attendance is rising again, as Alsop has tried to break through classical music's conservative image.
"Because they get lonely," Alsop said.
After performances, she'll take questions and talk about composers, like Tchaikovsky.
"He loved gardening, but he couldn't bear to cut anything back. So apparently his house looked like a complete jungle," she said. "And his symphonies sometimes get like that, don't they?"
Jane Marvine, who plays the English horn, said: "She's interested in demystifying the role of the maestro. She's a maestra."
A maestra who always knew this was where she was meant to be.
"Yeah, definitely. Are you kidding? This is better than what I wanted," she said. "Everyday is like Christmas for me."