On a late summer’s evening in 1985, she phoned the National Rifle Association headquarters and left a blunt message: “My name is Sarah Brady, and you’ve never heard of me, but I am going to make it my life’s ambition to try to put you all out of business.”

With that call, Mrs. Brady started down a road that would make her the public face of gun-control activism for a generation. Her husband, James Brady, was Ronald Reagan’s press secretary and was left paralyzed during an assassination attempt on the president in 1981. She was left to care for her husband through his long, and at times excruciating, convalescence. He died Aug. 4, 2014, at age 73.

But it wasn’t her husband’s shooting that led Mrs. Brady to call the NRA. The turning point for her activism came four years later, when their 6-year-old son, Scott, found what he thought was a toy gun and pointed it at his mother. She found to her horror that it wasn’t a toy but a fully loaded .22 similar to the one used to shoot her husband.

“The maddest I’ve ever been in my life,” she said of the incident that occurred during a visit to her husband’s hometown of Centralia, Ill. “I was livid.”

Mrs. Brady grew into a determined foe of the NRA, one of most powerful lobbying organizations in the country. She died Friday at a retirement community in Alexandria, Va. She had pneumonia, family spokeswoman Gail Hoffman said.

A lifelong Republican, Mrs. Brady reached out to a small organization, Handgun Control Inc., now the Brady Campaign to Prevent Gun Violence, and joined its efforts by writing letters to members of Congress as well as lobbying them in person on Capitol Hill. She was the driving force behind the gun-control legislation known as the Brady Bill, signed into law by President Bill Clinton in 1993.

To gain support for the Brady Bill, which required a waiting period and background check on all handgun purchases through federally licensed dealers, Mrs. Brady lobbied politicians, appeared on TV talk shows, wrote opinion pieces, and made speeches often to audiences packed with hostile NRA supporters.

Mrs. Brady became a political independent as her involvement in the gun-control campaign evolved. She backed Democrats such as former U.S. Sen. and 2004 presidential candidate John Kerry of Massachusetts, now the secretary of state, who supported gun control. In 1992, she supported Democratic presidential candidate Bill Clinton after President George H.W. Bush, a lifetime NRA member, refused to commit to supporting the Brady Bill.

Sarah Jane Kemp was born Feb. 6, 1942, in Kirksville, Mo. Her father worked for the FBI and moved the family to Alexandria. After graduating in 1964 from the College of William and Mary, she went on to become an assistant to the campaign director at the National Republican Congressional Committee in Washington. She met James Brady, and they married in 1973.

Survivors include a son, James Brady Jr. of Rehoboth Beach, Del.; stepdaughter, Melissa Brady Camins of Woody Creek, Colo.; and brother, William Kemp of Arlington.