Hanne Merriman, who started a career at Garfinckel's 21 years ago as part of the sales staff, yesterday was named president of the Garfinckel division of Garfinckel, Brooks Brothers, Miller & Rhoads Inc.

The promotion from her previous job as executive vice president of the company makes Merriman one of less than a handful of women in the country who preside over major department stores.

Merriman succeeds William C. Detwiler in the job. Detwiler, who has been president and chief executive officer at Garfinckel's, will continue as chief executive officer and will assume the newly created post of chairman of the division.

Detwiler will be continue to be responsible for financial, operations and personnel functions and plans for expansion, while Merriman will handle merchandising, sales promotion and direct mail marketing. With the exception of direct mail marketing -- which Merriman calls "a large and growing part of the operations" -- none of those responsibilities is new to her.

Merriman has set the fashion merchandising direction for Garfinckel's for several years, according to one close observer of the store's operations.

Merriman, who was born and educated in Denmark, went to work for Garfinckel's in 1960, three months after she arrived in the United States. From a job on the selling floor she moved to jobs as a buyer and several assignments as a divisional merchandise manager. In 1975 she was named vice president and general merchandise manager of the store. In January 1976 she became executive vice president.

Besides its landmark downtown store, Garfinckel's operatesd eight branch stores and two hotel shops. A new Garfinckel'sd store in Georgetown is scheduled to open in September, and Merriman has been involved in the planning for that.