"She's been one of the most influential lawyers in the country in terms of changing the courts' perception of the culpability of juvenile offenders," said Steven Drizin, clinical professor at Northwestern School of Law.
It's been strategic warfare, said Levick, who saw her chance in 2005, with the case Roper v. Simmons, which ended the juvenile death penalty.
"This was an opportunity we needed to seize and push it as far as we could - and we've had remarkable success," said Levick, who provided support in the case, including a brief and a 50-state survey of juvenile justice laws. "The world for kids in the justice system has changed at a phenomenal speed."
Levick had strong models for her single-mindedness. Her father was an oncologist; her mother, a psychologist who founded the first graduate-level art therapy program in the country at Hahnemann University Hospital.
"They figured out what interested them and pursued it to the greatest degree they could," she said.
But it was her service-oriented Friends Select School education - and the civil rights and antiwar movements dominating the news as she was coming of age - that set her path.
She knew in law school that she wanted to fight for disadvantaged individuals. And there was no public-interest law firm focused on children at the time.
Levick and three other Temple Law graduates founded Juvenile Law Center in 1975 in the spare room of a doctor's office. It was a community legal-service firm, run on a shoestring. They took on cases of kids in the child-welfare systems, in the juvenile and adult courts.
The founders struggled, and a few times went on unemployment.
But they fought for kids no one else cared about.
A decade ago, they took a call from desperate parents in Luzerne County, whose children were caught up in what became known as the "kids for cash" scandal: Judges were receiving kickbacks to send children, accused often of minor offenses, to juvenile placement.
Hillary Transue, 24, was one of those kids - jailed because of a satirical MySpace page about a school administrator she had created. She still sees Levick as a role model.
"She's been there for me since I was 15 years old," Transue said. "Children are so important to Marsha. She could do anything, but she chose this, and that's really profound."
Levick has also been a mentor for a generation of public-interest lawyers, such as Lauren Fine and Joanna Visser Adjoian, who started the Youth Sentencing & Reentry Project after finishing fellowships at the Juvenile Law Center.
"She showed us what it means to be a lawyer in a way that was inspiring and tangible," Fine said.
Suffolk University Law School Professor Jeffrey Pokorak, who was cocounsel on Montgomery, has worked with Levick on cases ranging from that of Omar Khadr, a juvenile Guantánamo Bay detainee, to Jalil Abdul-Kabir, a juvenile who was facing the death penalty. But he and Levick found room for humor and pop-culture references, including Buffy the Vampire Slayer. "We lovingly called it 'the Buffy Brief,' " he said.
But she took her work seriously. "We all sense injustice innately, but it takes a rare person to take that outrage and turn it into a passion for actual justice - a positive action," he said.
Bradley Bridge, a Philadelphia public defender, said he has often called on Levick to assist with cases over the decades. They have clashed over typefaces on briefs - and nothing else.
She brings perspective, he said. Where others see trees, she sees the forest.
"There's many different places along the way where you could be satisfied with a resolution. Marsha is relentless in pushing toward the best possible solution."
Levick, who is married to Tom Innis, a Philadelphia public defender, did leave Juvenile Law Center for a number of years to raise their two daughters, now 24 and 28, and to work for the National Organization of Women's legal arm and in corporate litigation.
But for the past 20 years, she's been back with more focus than ever.
In the last year alone, she's filed dozens of amicus briefs in support of various cases, and argued before a dozen state supreme courts and several U.S. courts of appeal.
Lou Natali, a Temple Law professor, called her "indefatigable."
"She's like a terrier with a bone," he said. "She just outworks her opponents."
Next up, Levick has her eye on two more potential Supreme Court cases. She's cocounsel on a civil rights case involving a 12-year-old boy strip-searched at a juvenile detention facility in Lancaster County. And she's filed an amicus in the case of a 10-year-old from California, arguing he was incapable of waiving his Miranda rights.
Given the Montgomery ruling - and President Obama's ban announced Monday on solitary confinement for juveniles - she thinks the momentum is on her side.
"Doing this work for 40 years, this is unprecedented, the challenging but also hopeful environment in which we're working," she said. "I feel like there is so much to do and there is so much that we could achieve."