Which department store head becomes romantically involved with Don in season 1?

The department-store head who becomes romantically involved with Don in season 1 is Rachel Menken.

Rachel is introduced in the pilot as the Jewish head of Menken’s department store, a wealthy New York retailer seeking an advertising agency to improve the store’s image. Don initially clashes with her because she rejects his coupon-based sales pitch and insists that Menken’s should attract more affluent customers.

Their professional conflict develops into mutual attraction. Rachel is unusually confident and direct with Don, challenging both his advertising assumptions and his views about women, marriage, and love. Their relationship is brief but important: she becomes one of the first women to see through parts of Don’s carefully constructed persona.

A common mix-up is confusing Rachel with Sylvia Rosen, Don’s later neighbor and lover, or Faye Miller, the market researcher he dates in season 4. Bobbie Barrett is another significant Don affair, but she is a comedian’s wife and manager—not a department-store executive.