"The United States Constitution has a 14th Amendment which states, and I paraphrase, neither the federal government nor any state shall deny to any person equal protection of the law," said Derr, now in his 80s and still practicing law in Boise.
But women were long denied equal protection, Sally specifically in 1967, when her son Skip died at age 16, leaving behind a small estate of several hundred dollars established by sally for skip's college education.
Sally, who was divorced, wanted to oversee her son's modest estate.
But a probate court in Boise ruled in favor of her ex-husband, Skip's father Cecil Reed, because Idaho law stated "males must be preferred to females" if more than one person claims responsibility.
Idaho's rule was not unusual at the time: similar statutes existed in Arizona, South Dakota, Wyoming and the District of Columbia.
Outraged, Sally Reed turned to attorney Allen Derr.
Derr appealed to Idaho's Fourth District Court, which overturned the probate decision and After all, she was the one who set up her son's estate.
But Cecil Reed appealed the Fourth District Court's ruling.
Then the case landed at the Idaho Supreme Court, where justices ruled against Sally Reed saying in general "men are better administrators than women and that nature itself had established this distinction."
So Derr reached out to the United States Supreme Court. In March 1971, the high court agreed to hear the case.
"They accepted it," Derr said. "And they only accept, out of thousands of cases, they only accept for argument two or three hundred a year."
And Derr accepted a new member on his legal team to help write the brief.
Ruth Bader Ginsburg, destined to become the second woman appointed to the United States Supreme Court, was intrigued by reed versus reed.
"Justice Ginsburg at the time was a law professor at Rutgers University," Derr said. "She also had become involved with the American Civil Liberties Union."
But in October 1971, it was Derr alone who faced the seven supreme court justices...all men back then.
Derr had 30 minutes to make his case that a state such as Idaho simply cannot discriminate against people based on their gender.
"My knees were shaking when I stood up," Derr recalled. "Warren Burger said, the way it starts is this, he said, Mr. Derr, you may proceed."
And the way it ended on Nov. 22, 1971 was a victory for Sally Reed.
The justices unanimously ruled Idaho's discrimination against Sally Reed based on her sex was the "very kind of arbitrary legislative choice...forbidden by the equal protection clause of the Fourteenth Amendment."
It was the first time the supreme court ever struck down any law on the grounds it violated the constitutional rights of women
"And I couldn't believe it," Derr recalled. "I was about as pleased as I've ever been. Sally was stoic. She was pleased as well. But she never showed much emotion. She got into this because of what she thought was fair."
Sally Reed died in 2002. She lived in a house on Vista Avenue until it was torn down in 1999.
There's a plaque at the site, honoring sally, a modest woman from Boise who simply demanded equal treatment.