Sandwiched between those years was 20th-century America’s longest run of mergers, acquisitions and divestitures, lasting from 1974 to 1989. Mature companies were seeking to rekindle their growth through new business combinations, and takeover artists were lunging after chances to reap big short-term gains.

Under Mr. Mullen, who died on Saturday at 83 in New Preston, Conn., Skadden Arps spotted the potential of mergers and acquisitions early on and then captured that business when New York’s more genteel corporate law firms were mostly shunning it.

The white-shoe firms "said that takeovers were a one-time blip," recalled John C. Coffee Jr., a law professor at Columbia, who in the mid-1970s practiced at one of those firms, Cravath, Swaine & Moore. "They said that Skadden was riding a short-term wave, and that it would be caught in a terrible crunch when it crashed and disappeared."

They misjudged. Advising on corporate takeovers proved to be a lasting and highly profitable business, attracting big corporate clients instead of scaring them off. Mr. Mullen and his colleagues plowed the earnings into Skadden Arps’s expansion and diversification into other areas of the law.

By the time merger mania died down in 1989, after the collapse of the junk-bond financier Drexel Burnham Lambert, Skadden Arps was no longer a small, specialized firm but a fixture in the top tier of the corporate bar, claiming about a third of the Fortune 500 as clients. It remains the world’s second-largest law firm by revenue, according to The American Lawyer, and the 11th-largest by number of lawyers.

"We like to think of our client as Wall Street," Mr. Mullen once told an interviewer.

His death, while en route to a hospital, was caused by a heart attack, his son-in-law, Thomas White, said.

Mr. Mullen joined the firm as a lawyer but ended up devoting most of his time to its business operations. Joseph H. Flom, the firm’s high-octane chairman at the time, reveled in takeover battles; Mr. Mullen excelled at building consensus among the firm’s powerful partners, making strategic decisions about growth and compensation and promoting the kind of community engagement that others at the young firm tended to overlook.