The decision left Garcia's mother Teresa Courtemanche nearly speechless.
"Relief, just relief that it's done," she said.
Prosecutors say Williams was the shooter Labor Day weekend 2008, when Garcia was gunned down in front of a friend's home.
Co-defendant Gene Allen Combs was with Williams. He is being tried separately.
Prosecutors argued the two men mistook the young politician for a drug dealer who owed them $50.
"These are the types of people that he tried to help and they're the ones that murdered him; he would've given them $50, any of us would've given them $50," Courtemanche said.
"So many people loved him, what he was doing for the city as a young politician, stopping crime, helping youth," Garcia's father Matt Garcia Sr. said.
The verdict comes after the jury of eight men and four women asked for witness testimony to be read back to them several times before settling on Williams' guilt.
"I never got to ask him, 'Why did you kill my grandson?' I wanted to ask him that," Garcia's grandmother Chris Garcia said.
Jury deliberations for Combs just began and will resume next week. Williams will be sentenced July 19.