William Bain Jr., Business Consultant and Romney Mentor, Dies at 80

William W. Bain Jr., who founded the business consulting firm Bain & Company and was an early mentor to Mitt Romney, died on Tuesday at his home in Naples, Fla. He was 80.

His wife, Ann Dean Bain, said the cause was complications of Alzheimer’s disease.

A native Tennessean who started his professional life as a development officer at his alma mater, Vanderbilt University, Mr. Bain at first seemed an unlikely candidate for the role of chief executive. Nevertheless, he made Bain & Company a fearsome competitor to McKinsey & Company, the Boston Consulting Group and Booz-Allen Hamilton. Along the way, he opened a venture-capital investing firm, Bain Capital, which remains a leader in its field.

The Boston-based Bain & Company worked with some of the biggest companies in the United States, among them Texas Instruments, Black & Decker, Monsanto and Wells Fargo.

“Chief executives have come to the very strong feeling that a small number of decisions, regarding a small percentage of policies they deal with, in the end determine 90 percent of their success or failure,” Mr. Bain said in a 1979 interview with The New York Times. “They are swamped with studies and information.”

“Corporate strategy,” he added, in a pitch for his own line of work, “allows them to isolate the small numbers of decisions that are going to have an impact. This makes it possible to manage big diversified companies.”

Bain & Company at one point said that for every dollar it charged a client, it increased that client’s profits by a factor of 5 to 10.

William Worthington Bain Jr. was born on July 30, 1937, in Johnson City, Tenn., to William and Ruby Worthington. He attended East Tennessee State University for two years before transferring to Vanderbilt and graduating in 1959 with a degree in history.

He was working as Vanderbilt’s director of development when Bruce Henderson, a fellow alumnus who had been impressed with Mr. Bain’s success in raising money for the school, made a compelling offer: to join his own young company, the Boston Consulting Group. Mr. Bain accepted, and moved from Nashville to Boston.

Once there, he quickly advanced, generating big revenues and earning respect from the firm’s clients.

He founded his own company in 1973. Before long, he was poaching both clients and talent from his old firm. “It was war,” Mr. Henderson later said in an interview.

Mr. Romney was one of Mr. Bain’s most significant hires. Mr. Romney, a future governor of Massachusetts and the 2012 Republican presidential nominee, joined Bain & Company from the Boston Consulting Group in 1977. A recent graduate of the Harvard Business School, where he had been at the top of his class, Mr. Romney later helped found Bain Capital, where he was managing general partner.

“It’s hard for me to imagine my life and career without Bill Bain’s mentoring,” Mr. Romney said in a statement after Mr. Bain’s death. “He hired me, taught me, and gave me the opportunity of a lifetime. His vision and confidence made Bain Capital possible.”

During his unsuccessful campaign to deny President Barack Obama a second term, Mr. Romney fell under criticism for using controversial tax-avoidance strategies on certain investments that he still held through Bain Capital.

A father of four, Mr. Bain set a work schedule for himself that enabled him to arrive home in time to read bedtime stories (and often to catch “Cheers” on TV). As coach of his children’s Little League teams, his son Alexander remembered, he occasionally arrived on the field at the last minute, still wearing his pinstriped suit.

By the early 1990s, Mr. Bain had stepped away from his management roles at Bain & Company and Bain Capital. He proceeded to found a small private-equity investment firm, Bain Willard Companies, with Ralph R. Willard, under which he became an owner and director of a yacht-building company.

In addition to his wife and his son Alexander, Mr. Bain is survived by three other children, William, Adam, and Samantha; three grandchildren; and a brother, Larry.