When police arrived two minutes later Martin lay face down, dead. Zimmerman had blood on his face and the back of his head and his back was wet. Questioned, then released by police, he was charged with murder six weeks later. He pleaded self-defence. Prosecutors painted him as a vigilante.
Initially just another crime statistic, the case became a cause celebre after Martin's parents demanded an investigation. Embraced by social media, it became a lightning rod for disparate views about crime, gun laws and racism.
As civil rights activists weighed in - the Reverend Al Sharpton urged protesters to join "justice for Trayvon" rallies in 100 cities across the US tomorrow - national attention focused on Florida's "stand your ground" gun law, and how whites were more likely than blacks to be freed if the law was invoked.
The lax Sanford police investigation evoked the segregationist Jim Crow era, as did claims of racial profiling. In a nation where violent crime is disproportionately committed by black men, who are also more likely to be incarcerated, it seemed inevitable the case would become a barometer for race relations.
The Lawyers' Committee for Civil Rights Under Law called the verdict "a tragic miscarriage of justice". The Obama Administration has proceeded cautiously. Attorney General Eric Holder said Martin's death was tragic and unnecessary and agreed Florida's stand your ground law could escalate violence.
But he warns that a federal case demands a "very ... high bar".
Doing so would mean proving Zimmerman was guilty of a hate crime - that he was motivated by racism and shot Martin dead because he was black.
That would be hard to prove without witnesses, video or other evidence. But America's first black Attorney General and its first black President, Barack Obama, who said last year that if he had had a son, "he'd look like Trayvon", are feeling pressure from the NAACP and others angered by the verdict.
Like the Rodney King and O.J. Simpson cases in the 1990s, Martin's death has become a litmus test on America's race divisions, despite Judge Deborah's Nelson's admonishment to jurors - five white women and one Latina who were initially split over Zimmerman's guilt, with half believing he should be acquitted - that the phrase "racial profiling" be banned from her courtroom.
"We went through this kind of surreal trial where the judge blocked all discussion of racial profiling," said NAACP president Benjamin Jealous.
The racial dynamics of Sanford seemed to "have come into play".
Speaking to National Public Radio, Jealous compared the Zimmerman verdict to a pivotal moment in the civil rights struggle, where two white men were acquitted of kidnapping and killing black teenager Emmett Till in 1955.
Since the verdict, more than a million people have signed a petition on the NAACP website demanding civil rights charges be filed, even as the black lobby group prepares to celebrate the 50th anniversary, next month, of the Great March on Washington, one of the biggest rallies for jobs and civil rights in US history, climaxed by Martin Luther King Jr's "I have a dream" speech.
That dream of a colour-blind America remains distant for many blacks, and the Zimmerman verdict could be a catalyst for wider grievances - unemployment, economic hardship and persistent discrimination - that mock Obama's promise of hope.
Black anger intensified in June when the US Supreme Court axed a clause of the 1965 Civil Rights Act.
It had demanded six states, mostly from the old Confederacy, obtain federal consent before changing voting laws, a bid to stop them re-enacting poll taxes and literacy tests that disenfranchised blacks.
During the 2012 election several states tried to pass voter ID laws, seen as a move to restrict voting by blacks and other minorities who usually vote Democrat. Yet Chief Justice Roberts said "that problem was solved".
Back in the 'hood, problems remain. On Monday a mob broke away from a protest in the Crenshaw district of Los Angeles, setting fires and trashing cars.
Some of the worse modern race riots - in LA's Watts district in 1965, nationwide in 1968, and in LA in 1992 after police were acquitted of beating Rodney King - erupted in long, hot summers like this one.
Despite apocalyptic warnings by pundits, protests against the Zimmerman verdict have been overwhelmingly peaceful.
For many blacks, racial prejudice is a constant, 150 years after Lincoln's Emancipation Proclamation ordered slaves be freed.
While demonstrators wore hoodies in solidarity with Martin, anxious black parents have counselled against hoodies and offer more sobering advice to children.
"I take my hat off, I take my sunglasses off, I put them on the passenger's side," actor LeVar Burton told CNN. "I roll down my window, I take my hands, I stick them outside the window and on the door of the driver's side because I want that officer to be as relaxed as he can be. "
Zimmerman may now appreciate such fears. A letter published on Facebook by Alex Fraser, a young black man, said Zimmerman would spend the rest of his life feeling "what it's like to be a black man in America", called names, stared at, avoided and denied jobs.
"I bet you never thought that by shooting a black male you'd end up inheriting all his struggles."
Stand Your Ground law
Stand Your Ground law as it was used in the Zimmerman trial:
• Stand Your Ground allows a law-abiding citizen to "meet force with force, including deadly force" if he reasonably feels threatened in a confrontation
• The National Rifle Association-drafted law was passed by the Florida legislature in 2005
• It made two major changes to homicide cases:
• It changed standard jury instructions, which previously held that a person had a duty to retreat by using "every reasonable means".
• It gave prospective defendants the right to immunity from prosecution. To make the immunity determination, the courts established pre-trial Stand Your Ground hearings.
• Zimmerman waived his right to the Stand Your Ground immunity hearing, but he was afforded the protections of the law, which is embedded in Florida's self-defence laws.
• Its language, tailored to the trial's jury instructions, says: "If George Zimmerman was not engaged in an unlawful activity and was attacked in any place where he had a right to be, he had no duty to retreat and had the right to stand his ground and meet force with force, including deadly force if he reasonably believed that it was necessary to do so to prevent death or great bodily harm to himself or another or to prevent the commission of a forcible felony."
