The signature burgers -- which normally cost $5.50 -- will be available for 80 cents from 12 p.m. to 8 p.m. at all of the chain's locations in California and Nevada.
The main celebration being held at Original Tommy's first restaurant, the iconic "Shack" located at the intersection of Beverly and Rampart boulevards in the Westlake district. Giveaways entertainment, including a DJ, was part of the festivities.
In an interview at the landmark location, a spokesman said he believes Original Tommy's is "iconic to all the Angelenos, simply because this location has been 24 hours a day, seven days a week since the opening.
"So it's always available -- pre-Dodger games, post-Dodger games, late nights, after the clubs, we're always here," he said. "So I think we've just been a staple of the community."
The Belmont High School local marching band got the party started with a late-morning performance.
"It's hard to believe that we're here," said Dawna Barnal, CEO of Original Tommy's and granddaughter of the chain's founder. "We just want to do something to thank our loyal who've been with us. We wouldn't be anything without them.
"My grandfather was always big on that -- the customer's always right," she said.