The royal couple, in the western US state for three days after a nine-day tour of Canada, were also set to use their star power to promote British actors and filmmakers in Hollywood.
After arriving in Los Angeles on Friday, they spent the night at the residence the British consul-general in Los Angeles, who also hosted a reception for them on their first evening in town.
On Saturday they were to travel an hour up the Pacific coast to the Santa Barbara Polo and Racquet Club, for a charity match in aid of the American Friends of The Foundation of Prince William and Prince Harry.
Security was tight at the prestigious polo ground, where sniffer dogs were checking spectators who began arriving from early morning for the afternoon game, as helicopters clattered overhead.
A heavy morning fog shrouded the mountains behind the ground -- a common feature along the ocean front -- but the sun was expected to burn them off in time for the royal match.
The 29-year-old prince, second in line to the British throne after his father Prince Charles, was to demonstrate his horsemanship while Catherine was to present the prize to the winning players.
Tickets for the one-off event have been fetching thousands of dollars, and spectators were hoping to get their money's worth, whether or not William wins -- or takes a tumble.
But the prince, who follows in his father's and grandfather's footsteps in playing polo, may have to watch out for a more aggressive American approach to a sport perceived as an upper class in Britain.
"There’s a huge difference," said Ebe Sievwright, a player and coach with the California Polo Club who is also a British-born actor who says he has played with William.
"Polo in England is very Argentine influenced, and in America there is a great history of cowboys and riding from their own history out in the Wild West," he told CNN.
"It will be interesting for William out in Santa Barbara to play in a game where it's really flowing like a pinball game," he added.
Whatever the outcome on the polo field, the couple will have to pick themselves up late Saturday for a black-tie dinner hosted by the British Academy of Film and Television Arts (BAFTA) in downtown Los Angeles.
William is BAFTA's president, and the aim of the event will be to give up-and-coming British actors a chance to meet Hollywood movers and shakers.
Officially, the Californian mini-tour will aim to "contribute to deepening and strengthening the multi-layered links between the United States and the United Kingdom," according to a British official briefing.
Unofficially, all eyes are on who they will meet during the barely 48-hour stay in California.
Britain is well represented here, and there has been much speculation that they could squeeze in meetings with David and Victoria Beckham -- guests at their April wedding -- or Elton John, a close friend of William's mother Diana.
On Sunday, they will visit "Skid Row" in downtown to visit the Inner-City Arts School, a non-profit academy that provides free classes in visual and performing arts to children from poor neighbourhoods.
Later Sunday they will attend a job fair for military veterans at Sony Studios in Culver City, before leaving Los Angeles to return to the United Kingdom after their 12-day North American trip.
Copyright AFP (Agence France-Presse), 2011