William Russell Kelly founded Russell Kelly Office Service in 1946. The firm provided services performed at Kelly's offices. Later, clients requested that the work be handled at their locations, and business of temporary staffing was created.
According to Wikipedia, "The temporary workers, who were usually female, became known as 'Kelly girls.' In recognition of this, the name of the company was changed to Kelly Girl Service Inc. in 1957. The term 'Kelly girl' became a widely used term for a temporary worker, regardless of the worker's company affiliation or gender. In 1966 the name of the company was changed to Kelly Services Inc., in order to reflect an expanding range of services."