In 1960, while still in his twenties, Weill and three partners started a small brokerage: Carter, Berlind, Potoma & Weill. Over the next 20 years, Weill led the brokerage through 15 acquisitions, building it into the financial powerhouse Shearson, the second largest company in the securities industry. He sold Shearson to American Express in 1981 for $930 million. Weill became President of American Express, whose directors specifically hoped he would be able to turn around their failing insurance operation, Fireman's Fund. Although Weill succeeded in this goal, he became frustrated with the corporate culture at American Express and attempted unsuccessfully to buy Fireman's for himself. In 1985, he resigned from American Express.
At age 53, Weill, although wealthy, was out of work and thwarted in his efforts to find a job. In 1986 he traveled to Minneapolis to persuade Control Data to spin off its subsidiary, Commercial Credit, in an IPO (initial public offering) worth $850 million. Control Data sold 82 percent of the company to the public, Weill took over as CEO, investing $7 million of his own money. Besides Commercial Credit's lending operation, Weill had acquired its subsidiary, a property and casualty insurance company called Gulf Insurance.
Over the next two years, Weill's Primerica Corp. absorbed the consumer lending operations of Barclays American/Financial, and acquired receivables and branches from Landmark Financial Services.
In 1992 alone, Primerica raised $625 million by selling non-strategic assets. Weill made a decisive move, buying 27 percent of Travelers Insurance for $722.5 million. Sanford Weill earned $67.6 million that year, most of it from stock options. He was the second highest-paid executive in America.
In 1996, Weill acquired the property and casualty operations of the insurance provider Aetna for $4 billion. The insurance companies were reorganized into separate life, term, and property-casualty operations. Smith Barney Shearson played a major role in financing the merger of Viacom Inc. with Paramount Communications. Travelers' 1996 profits made it number 32 of the Fortune 500 list of America's most successful companies. The original shares of Commercial Credit appreciated tenfold in the decade after Weill bought them.
Throughout his career, Sanford Weill ran his businesses like an owner. He avoided consultants and got to know his company by getting to know people at all levels of the business. All of Travelers' employees were encouraged to own stock in the company. Senior managers received up to 25 percent of their pay in stock, which they were not allowed to sell for two years.
In September 1997, Travelers paid $9.1 billion in stock to acquire Salomon, Inc., the parent company of the investment bank Salomon Brothers. Weill merged Salomon with Smith Barney to create the second largest securities firm in the world. In April 1998, Weill announced the biggest coup of all: Travelers Group would merge with Citicorp, the parent company of Citibank, to create Citigroup, Inc. At the time, Citicorp was the world's largest supplier of credit cards, and Citibank was the second largest bank in the United States. On the morning the planned merger was announced, the two companies were valued at about $70.6 billion. By the end of the day, the value of their combined stock had jumped to $83.6 billion.
In 1999, both houses of Congress passed the Gramm-Leach-Bliley Financial Services Modernization Act by overwhelming margins, and President Clinton signed the act into law. The measure won bipartisan support by reforming certain discriminatory banking practices, such as the "redlining" of low-income neighborhoods, while repealing the Glass-Steagall restrictions on the intermingling of commercial banks and investment banks, and of banks with insurance companies. Gramm-Leach-Bliley permitted the completion of the Citigroup merger, and set off a wave of similar combinations in the financial services industry. Some economists believe this deregulation contributed to the credit crisis that engulfed the world economy less than a decade later.
Few considered this possibility in 1999, as Citigroup became the largest financial services company in the world, with 100 million customers in 100 countries. At first, Sanford Weill served as Co-Chairman and Co-CEO with John Reed, but in 2000 Weill became the sole Chairman and CEO of Citigroup. Under Weill's leadership, Citigroup achieved unprecedented growth, earning $13 billion in 2001. New subsidiaries were acquired or created all over the world, particularly in Asia and the newly liberated economies of Eastern Europe. Weill stepped aside as CEO in 2003 and retired from the Chairmanship in 2006.
Sanford and Joan Weill continued their commitment to philanthropy. , In 2016 they announced a $185 million gift to the University of California at San Francisco to create a new institute for the study of neuroscience. The Weill Institute for Neurosciences will focus on research into the causes and cure of Alzheimer's and Parkinson's diseases, as well as other forms of dementia, sleep disorders, chronic migraine, stroke and paralysis. The center will consolidate existing departments of the university and add 45 new basic research labs, integrating the study of all brain-related conditions, including psychiatric disorders.