Forrest E. Mars Jr., a billionaire scion of the reclusive family that satisfied America’s sweet tooth with the Milky Way candy bar and M&M’s and who helped build Mars Inc. into the world’s largest confectionary company, died on Tuesday in Seattle. He was 84.

The company, which he inherited with his brother and sister in 1973, said the cause was a heart attack.

Mr. Mars and his brother, John, were co-presidents of the company, which sold about $1 billion worth of candy when their father turned over control. By the time Forrest Mars Jr. retired from active management in 1999, it was an $18 billion-a-year company selling Snickers, Uncle Ben’s Rice and Pedigree pet food. Since its acquisition in 2008 of the Wm. Wrigley Jr. Company, the chewing gum manufacturer, Mars reports sales of $35 billion a year and has 80,000 employees worldwide.

Mr. Mars played an early role in the company’s global expansion, was group vice president for confectionary operations and became co-president with his brother in 1975 as Mars tapped new markets from Africa to Russia.