Now, a century later, the city is celebrating that moment in its past by giving the Ferry Building clock tower a new, illuminated look.
The tower's been decorated with the numbers "1915" and more than 1,000 lights.
Behind a locked door, seven stories above the ground is the western face of the Ferry Building clock.
Jim Phelan, a third-generation steeplejack was in charge of building, hoisting up and securing the 1915 sign to the Ferry Building tower, and securing the strings of lights that outline the building.
Phelan explained that "a steeplejack is someone who climbs hard-to-access places - flagpoles, church steeples, clock towers."
The numbers are 11 feet, 6 inches high, and about 26 feet wide. They each weigh about 150 lbs.
The numbers and lights commemorate the 100th anniversary of the Panama Pacific International Exposition, by re-creating the way the Ferry Building tower looked a century ago.
"It was lit up to welcome the world," Donna Huggins, a historian of the fair, explained of the Ferry Building lights a century ago.
In 1915, the Panama-Pacific International Expo put San Francisco on the world stage. About 20 million people came to San Francisco over a 10-month period to see it.
"It's really a turning point in the history of our city," Huggins said. "The exposition celebrated the re-emergence of San Francisco after the '06 earthquake."
It was also a turning point for some of the millions of people who attended, who toured the dozens of palaces and buildings built over 635 acres of the city in the area that is now the Marina District.
Donna Huggins has made it her life's work studying the history of the expo, collecting antiques and artifacts from the period, from fair-goer ticket books to a couple of the glass cut stones that adorned the exterior of some of the Expo's palaces.
Huggins' grandparents got engaged at the Expo.
"That was where they courted. They were holding hands, strolling, learning about each other," Huggins said. She acknowledging that, in a way, she owes her very existence to the fair.
"I think that's very true and very possible. It's very personal," Huggins said with a laugh.
Huggins led a fundraising effort to come up with the $88,000 dollars in private money needed to light up the ferry building.
Her drive -- and Phelan's hard work -- helped make this tribute to San Francisco's history happen.
"This is the city. The city that knows how. We haven't lost that spirit," Huggins said.
The new Ferry Building lights will be turned on every night from now until Dec. 4, which was the date the Expo ended 100 years ago.