Orange County benefited from the July 4, 1904 completion of the Pacific Electric Railway, a trolley connecting Los Angeles with Newport Beach and Santa Ana. The link made Orange County an accessible weekend retreat for celebrities of early Hollywood. It was deemed so significant that the city of Pacific City changed its name to Huntington Beach in honor of Henry Huntington, president of the Pacific Electric and nephew of robber baron Collis Huntington. Transportation further improved with the completion of the State Route and U.S. Route 101 (now mostly Interstate 5) in the 1920s.

Agriculture, such as the boysenberry which was made famous by Buena Park native Walter Knott, began to decline after World War II but the county's prosperity soared. The completion of Interstate 5 in 1954 helped make Orange County a bedroom community for many who moved to Southern California to work in aerospace and manufacturing. Orange County received a further boost in 1955 with the opening of Disneyland.

In 1969, Yorba Linda-born Orange County native Richard Nixon became the 37th President of the United States.

In the 1980s, the population topped 2 million for the first time. Orange County had become the second largest county in California.

A spectacular investment fund melt-down in 1994 led to the criminal prosecution of Orange County treasurer Robert Citron. On December 6, 1994, Orange County declared Chapter 9 bankruptcy, from which it emerged in June 1995; this was the largest ever municipal bankruptcy in the U.S. The county lost about $1.6 billion through high-risk investments in derivatives.

In recent years, the county has been characterized by conflict between the older northern and newer southern cities over development, the building of new toll roads, and a recently defeated proposal to build an international airport at the former El Toro Marine Corps Air Station that would have reduced operations at the existing John Wayne Airport.