Sybrina Fulton issued a call to action against "stand your ground" self-defense laws and told the audience that she believes God is using her and her family to make a difference.
She said she was heartened that her message is getting through when, on the way to the civil rights organization's annual conference, she handed a card bearing her son's photograph to a driver and he told her, "That's my son, too."
"So my message is coming across, and it's coming across the right way," Fulton told attendees.
The message, she said, is to use her son's story to prevent other children from dying the same way.
"My message to you is please use my story, please use my tragedy, please use my broken heart to say to yourself, 'We cannot let this happen to anybody else's child,'" she told the audience.
Fulton supports an investigation into whether her son's killer could be charged under federal civil rights laws and she attacked Florida's "stand your ground" has having "prevented the person who shot and killed my son to be held accountable and to pay for his awful crime."
A jury acquitted George Zimmerman of all criminal counts in the slaying of the unarmed 17-year-old. Martin was shot and killed during a confrontation with Zimmerman that has spawned a national debate about "stand your ground" laws in Florida and other states.
Earlier Friday, a juror told ABC's "Good Morning America" that she feels like she let down Martin's parents. Juror B29 said in the taped interview that the panel couldn't find enough evidence under Florida law to convict the neighborhood watch volunteer in Martin's death last year.
The National Urban League's conference runs through Saturday at the Pennsylvania Convention Center.