But Schwarzenegger, who has an estimated net worth of $300 million , didn't make his first million in entertainment at all.

His big Hollywood break came in 1982, with his role in "Conan the Barbarian," and since, his films have grossed more than $3 billion. He also served as California's governor from 2003 to 2010.

Arnold Schwarzenegger's career has been impressively diverse. Before focusing on acting, the Austrian-American dominated competitive bodybuilding, winning five Mr. Universe titles and seven Mr. Olympia titles.

"I did not rely on my movie career to make a living," he tells author Tim Ferriss in "Tools of Titans."

"That was my intention, because I saw over the years, the people that worked out in the gym and that I met in the acting classes, they were all very vulnerable because they didn't have any money, and they had to take anything that was offered to them because that was their living. I didn't want to get into that situation."

Instead, he turned to real estate in the 1970s. Using the money he made from bodybuilding, Schwarzenegger put a down payment on an apartment building. He quickly started benefiting from a "magic decade," he tells Ferriss: "Buildings that I would buy for $500K within the year were $800K and I put only maybe $100K down, so you made 300% on your money.

"I quickly developed and traded up my buildings and bought more apartment buildings and office buildings on Main Street down in Santa Monica and so on. … I became a millionaire from my real estate investments."

Thanks to the income he generated through his side hustle, Schwarzenegger had the financial freedom to focus on his acting career, and the rest is history.