Williams was commissioned an ensign in the United States Navy in May 1987. After a six-month temporary assignment at the Naval Coastal System Command, she was designated a Basic Diving Officer. She next reported to the Naval Air Training Command, where she was designated a Naval Aviator in July 1989. She received initial H-46 Sea Knight training in Helicopter Combat Support Squadron 3 (HC-3), and was then assigned to Helicopter Combat Support Squadron 8 (HC-8) in Norfolk, Virginia, with which she made overseas deployments to the Mediterranean, Red Sea and the Persian Gulf for Operation Desert Shield and Operation Provide Comfort. In September 1992, she was the Officer-in-Charge of an H-46 detachment sent to Miami, Florida, for Hurricane Andrew relief operations aboard USS Sylvania. In January 1993, Williams began training at the U. S. Naval Test Pilot School. She graduated in December, and was assigned to the Rotary Wing Aircraft Test Directorate as an H-46 Project Officer and V-22 chase pilot in the T-2. Later, she was assigned as the squadron Safety Officer and flew test flights in the SH-60B/F, UH-1, AH-1W, SH-2, VH-3, H-46, CH-53, and the H-57. In December 1995, she went back to the Naval Test Pilot School as an instructor in the Rotary Wing Department and as the school’s Safety Officer. There she flew the UH-60, OH-6, and the OH-58. She was then assigned to USS Saipan as the Aircraft Handler and the Assistant Air Boss. Williams was deployed on Saipan in June 1998 when she was selected by NASA for the astronaut program. She has logged more than 3,000 flight hours in more than 30 aircraft types.