Donald Bren owns one-fifth of Orange County, an area five times the size of Manhattan. Pool/Getty Images; AP/Reed Saxon; Tanza Loudenback/Business Insider When it comes to real estate wealth, one Donald is the clear winner.

California native Donald Bren is the wealthiest real estate baron in America, with an estimated net worth of nearly $17 billion, according to Bloomberg's Billionaires Index, nearly six times President Trump's fortune.

Unlike Trump, whose wealth is inherited according to Bloomberg, Bren is a self-made mogul who turned a $10,000 bank loan into a multi-billion dollar empire.

Bren's privately-held real estate investment company, Irvine Company, is the largest landowner in California.

Irvine Co.'s portfolio of properties exceeds 110 million square feet and includes office buildings, apartments, marinas, and hotels, most of which is located in picturesque Orange County. Bren also has a footprint in Trump's native New York City as the majority owner of the New York Met Life building.

In total, Bren owns one-fifth of Orange County, an area five times the size of Manhattan, according to Bloomberg.

At 85 years old, Bren, a former US Marine, is still running the show as chairman of Irvine Company — here's the story behind his success.