Every so often, the FBI will do a "Celebrating Women's History" post where they talk about the three first female special agents. Two of them were hired in the early 1920s and promptly "resigned" when a new FBI Director took office. But in 1924, at the repeated urging of Congressman Graham of Pennsylvania, 45-year-old Lenore Hoover was promoted from "special employee" (aka investigative assistant) to "special agent".
This is the FBI's standard blurb about her:
In November 1924, Lenore Houston, an employee in Philadelphia, became the first and only female special agent hired by Director Hoover. While serving in the Philadelphia office, Miss Houston received excellent performance ratings and was earning $3,100 a year by April 1927. She resigned in 1928, shortly after being transferred to the Washington Field Office.