In the late 1960s, Ethel Lawrence and her husband lived with their children in a two-story house on an acre of land in Mount Laurel, N.J. Her family had lived in the township for generations, and she wanted her children to be able to live there, too. But the area was changing, fast.

The township, near Philadelphia, had been home to Black families like the Lawrences since before the Civil War, when it was a stop on the underground railroad. Then, in the decades after World War II, the federal government financed the construction of highways and tracts of single-family homes. White families fleeing cities like Camden, N.J., and Philadelphia tripled Mount Laurel’s population between 1950 and 1970.

When the township sought to condemn an area of dilapidated homes, where some of the town’s Black residents lived, Ms. Lawrence jumped into action. She and her husband owned their home, but Ms. Lawrence was concerned that other Black or Latino residents would be forced out. With a local minister and a community group, she came up with a plan to build 36 new affordable units, even securing $35,000 in state grants to study it. But there was a big hurdle: Mount Laurel’s planning board said the project wasn’t allowed under its zoning code.

Ms. Lawrence was furious. She and other Mount Laurel activists connected with young lawyers from Camden Legal Services who were looking for a test case to address what they saw as de facto segregation in zoning codes. They filed a lawsuit against the township in 1971, and Ms. Lawrence — a church pianist, Girl Scout leader, mother of nine and tireless activist — became the lead plaintiff in a suit that would eventually alter the state’s housing landscape.

In 1975, the state Supreme Court court ruled in favor of Ms. Lawrence and the other plaintiffs. It said that Mount Laurel had to change, but the decision went much further, interpreting the state’s constitution to require each of New Jersey’s 564 municipalities to build their fair share of affordable housing.