Relegated to the bench before the Olympics and forced into action by injury, Lloyd delivered as the U.S. claimed Olympic gold for the fourth time in five opportunities.
She scored on a header in the eighth minute, and with her right foot in the 53rd, in front of 80,203 fans, an Olympic record for a women's soccer game. A world record 90,185 spectators watched the U.S. women win the 1999 World Cup at the Rose Bowl in Pasadena.
The Japanese beat the U.S. on penalty kicks at last year's World Cup, which left many Americans stunned.
They took advantage of their second chance, just Lloyd took advantage of her opportunity.
Lloyd was benched during exhibition matches leading up to the Olympics. Lauren Cheney started instead. Center midfielder Shannon Boxx, a defensive anchor, injured a hamstring in the 4-2 victory against France. Lloyd took on a defensive role that wasn't really her style.
When Cheney got hurt, Boxx returned to the center and Lloyd was moved forward.
The U.S. women won all six games in this tournament, coming from behind in two.
The Americans posted three consecutive shutouts before facing Canada, which threatened to snap the USA's 26-game unbeaten streak in the series.
In Monday's semifinal, Cal grad Alex Morgan scored in the second overtime period to lift the U.S. to a dramatic 4-3 victory over Canada.
Megan Rapinoe scored twice and Olympic veteran Abby Wambach converted a penalty kick in the 80th minute to set up overtime.
In the 123rd minute, the match on the verge of going into a shootout, Morgan headed a cross from Heather O'Reilly into the back of the net to give the U.S. its first lead of the match.
Christine Sinclair, who scored all three goals, gave the Canadians the early advantage, scoring in the 22nd minute.
Stanford grad Kelley O'Hara, the left outside back, continued her streak of playing every minute of every match, one of three U.S. players to do so.
She was forced to step off the field in overtime after a collision forced her to receive treatment.
O'Hara sparked several attacks up the left flank, and Cardinal grad Rachel Buehler, who was replaced in the second overtime shortly after landing awkwardly in another collision, was a stalwart in central defense.
The Americans outshot the Canadians 18-9, though both countries got seven shots on goal.