Byrne died Thursday at his home in Etna, N.H., according to Robert E. Snyder, a family spokesman. The cause was prostate cancer.
In his letter to Berkshire shareholders reviewing 1980, Buffett credited Byrne's "managerial brilliance" with resuscitating Geico after his arrival in 1976. "There aren't many Jack Byrnes in the managerial world, or Geicos in the business world," Buffett wrote.
At the time of the letter, Buffett's Berkshire Hathaway Inc. owned one-third of Geico. Buffett bought the remaining two-thirds in 1996 for $2.3 billion and made it a unit of Berkshire. By then, Byrne had left Geico for White Mountains Insurance Group Ltd.
"Jack's performance in reviving Geico from near-bankruptcy was truly extraordinary, and his work resulted in enormous gains for Berkshire," Buffett wrote in his letter summarizing 1985, noting Byrne's departure.
Byrne arrived in 1976 as president and chairman of Geico -- an acronym for Government Employees Insurance Co. -- after working at Travelers Corp. Geico was near bankruptcy, having posted a net loss of $126 million in 1975, according to a 1981 article in Forbes magazine.
Byrne shut about 100 offices, cut the workforce to about 4,000 from 7,000, quit the highly regulated auto-insurance markets of New Jersey and Massachusetts and raised rates by as much as 40 percent, Forbes said. By 1980 the company was back to health.
Byrne's survivors include his wife, Dorothy; sons John, Mark and Patrick; seven grandchildren; and his brother, James Byrne, according to the Rand-Wilson Funeral Home in Hanover, N.H.