About 35,000 to 40,000 people were expected for the start of the 80th winter season, which runs through July.
The first 5,000 kids under the age of 12 were expected to receive a free plush horse on opening day.
Legendary trainer Bob Baffert, who had three horses racing Monday in the park's Malibu Stakes, said he had been looking forward to this day.
"We run some really big races today," Baffert said. "Important races. It's like the last great one race of the year."
For more on opening day, watch Jade Hernandez's full report in the video player above.
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