Joined the Manhattan law firm of Skadden, Arps, Slate, Meagher & Flom in 1961, when it consisted of just 10 lawyers; when he stepped down as its de facto chief executive in 1994, it had more than 1,000, and was one of the world’s richest and most prominent firms. Born in Manhattan in April 8, 1928, the son of John and Elsie Mullen. His father was a New York State Criminal Court judge, and his mother was a librarian. Skadden Arps made him partner after his first year with the firm, and managing partner in 1967, putting him in charge of its business operations. Came from a Roman Catholic family and was educated by the Jesuits, first at the Loyola School on the Upper East Side of Manhattan, then at Georgetown, in the days when Catholics were still unwelcome in the largely Protestant white-shoe firms. After Columbia Law School, he spent nine years at the firm of Dewey Ballantine, only to be voted down for a partnership. He served on Georgetown’s board from 1982 to 1998 and was its chairman from 1985 to 1992. He was also once chairman of Orbis International, which provides eye care in developing countries. Mr. Mullen is survived by his wife, Cecilia, known as Billie; five children, Peter, Kirby White, Elaine Peer, Lucy Ball and Jeff; and nine grandchildren. Mr. Flom died in February at 87.