McDonald's says the law threatens business and restricts parents' ability to make choices for their children.
The measure will take effect in December 2011 if supervisors approve it again, after an expected veto by San Francisco Mayor Gavin Newsom.
Supervisors say the law would make San Francisco the first major city in the county to take this action to combat childhood obesity.
A similar ordinance has been approved in California's Santa Clara County, where it affected about a dozen restaurants.
