Johnston and Kelci Bryant finished second with 321.90 points, ending the country's diving medal drought that extended to the 2000 Sydney Games. It was the first-ever Olympic synchro medal for the Americans.
"Our curse is out of the way," said Bryant, who finished fourth in springboard synchro with a different partner in Beijing.
"Abby and I just kicked it off with this event and I think the rest of the team is going to come through," she said. "After 2008, all of us were really close and we needed to fine-tune our training."
Laura Wilkinson's gold on 10-meter platform in Sydney, where she upset the Chinese despite a broken foot, was the last U.S. medal. The Americans were shut out for the first time ever four years later in Athens and again in Beijing.
"This is a new beginning," U.S. high performance director Steve Foley said. "We've got to forget the great USA diving history because really that was 20-plus years ago. It wasn't just 12 years ago when Laura won because they only got one medal, so the glory days were gone a long time ago."
After receiving their medals — Bryant cried and Johnston smiled broadly on the podium — Bryant reminded Johnston to hold up her prize as they posed for a gaggle of photographers on the deck.
"Ahhhh!!! So happy for our girls!!" tweeted American David Boudia, who will compete in 10-meter individual and synchro. "Great start."