The story resonated with voters, who elected her Tuesday to fill a long-red House seat that Georgia Republicans held onto just last year.
"We've sent a strong message to the entire country," McBath tweeted Thursday after Rep. Karen Handel conceded. "Absolutely nothing - no politician & no special interest - is more powerful than a mother on a mission."
McBath's son, Jordan Davis, was slain at a Jacksonville, Florida gas station in 2012 by a white man who was angry over the loud music the black teenager and his friends had been playing in their car.
Michael Dunn used the "stand-your-ground" law in his defense, but was convicted and is serving a life sentence.
After her son's slaying, McBath became active in gun control advocacy. John Feinblatt, who heads the gun control group Everytown for Gun Safety, said McBath first came to them as a volunteer and eventually joined the staff and headed faith outreach efforts.
"We saw in Lucy a natural leader that people immediately started to look up to for her real devotion to honoring her son and ability to rally others," he said.
Over the years she's testified before Congress and spoken at rallies, often wearing a button with her son's photo on it. In the 2016 presidential race, as a member of "Mothers of the Movement," she endorsed Hillary Clinton. The Mothers group is a sisterhood of black women whose sons were shot to death by white men or police officers.
McBath decided to run for office after the February massacre at a south Florida high school, she said during a campaign rally. In running against Handel, McBath made gun control a key issue, sharing with voters her personal journey.
"I lost my son Jordan but I'm still his mother. And I still continue to mother him through making sure that I preserve the lives of other children like him," she said during a television ad.
The longtime Delta Airlines flight attendant also frequently mentioned her battle with breast cancer during the campaign as she attacked Handel on health care issues.
Her neophyte campaign for the suburban Atlanta seat former House Speaker Newt Gingrich held got a big boost from Democrat Stacey Abrams' efforts to increase turnout in the governor's race. McBath appeared on stage with Abrams and former President Barack Obama during a campaign event last week.
Handel conceded Thursday and offered McBath "good thoughts and much prayer for the journey that lies ahead for her." McBath's victory extends Democrats' strength in suburban districts.
In 2016, Rep. Tom Price cruised to victory without a serious Democratic challenge. But his resignation to join President Donald Trump's Cabinet as health secretary set off a fierce special election battle featuring Handel against Democrat Jon Ossoff. Handel won, leaving the appearance that the district was still fundamentally Republican.
But getting any kind of gun control measures signed into law, as McBath hopes, will be difficult if not impossible in the next Congress.
Republicans expanded their Senate majority Tuesday, and Trump remains a favored ally of the National Rifle Association. In his midterm campaign sweep, the president frequently delighted his boisterous rallies with promises to "protect the Second Amendment."
Another congressional race remained too close to call Thursday in the neighboring 7th District. Absentee ballots were still being counted in Gwinnett County, where four-term GOP incumbent Rob Woodall remains locked in a tight contest with Democrat Carolyn Bourdeaux, a college professor who outpaced the congressman in total fundraising.
Associated Press writers Bill Barrow in Atlanta; and Russ Bynum in Savannah, Georgia, contributed.
For AP's complete coverage of the U.S. midterm elections: http://apne.ws/APPolitics
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.