“In the days when we worked together, he would always turn out the lights,” says Richard Favretto, a partner at Mayer Brown, of his former Department of Justice colleague Donald Flexner.

Flexner worked late nights for the duration of his career, and with good reason. In 12 years with DOJ's Antitrust Division, where he rose to become deputy assistant attorney general and acting head, he oversaw major litigation such as United States v. AT&T, which ultimately led to the breakup of the telephone giant in 1984.

After leaving the DOJ in 1980, Flexner built a premiere antitrust practice at Washington, D.C.'s Crowell & Moring before joining a two-year-old litigation boutique called Boies & Schiller in 1999 as a name partner. His work since has helped power Boies Schiller Flexner into a 310-lawyer firm that ranks 82nd on the Am Law 100.