Gail J. Koff, who could be considered the silent partner in the national law firm Jacoby & Meyers, a sort of legal Wal-Mart for the middle class, died Tuesday in Manhattan, where she lived. She was 65.

The cause was complications of leukemia, her former husband, Ralph Brill, said.

Ms. Koff was not there in September 1972 when Stephen Z. Meyers and Leonard G. Jacoby, his former law school classmate at the University of California, Los Angeles, opened their first storefront office in Van Nuys. But her aspirations matched those of the founders, and six years later she became the third partner, though unidentified in the firm’s name, assigned to open the first New York office.

Recognizing that the rich can afford lawyers and that the poor have access to free assistance programs, Jacoby & Meyers focused on serving average people who could often not afford to hire a lawyer at prevailing rates.

The firm set off something of a revolution in the field by using mass-marketing techniques and charging flat fees for services. It opened walk-in neighborhood “legal clinics” staffed by general practitioners who had access to teams of specialists in areas like bankruptcy, real estate, personal injury, divorce and criminal law.