A military brat born in the late 1960s, Sarah MacKenzie had a troubled childhood; her mother abandoned her and her alcoholic father on the night of Mac's 15th birthday. Her father, Joseph MacKenzie, a non-commissioned officer in the Marine Corps, was physically abusive towards her mother, and emotionally and verbally abusive to Mac. As a partial result of this, Mac became an alcoholic in her mid-to-late teens and had no contact with her mother Deanne (Conchata Ferrell) until she reappeared at her father's deathbed. She also learns at this time that her large white mutt, named Ruggles, was abandoned at a boarding house during her mother's travels after fleeing from her abusive father. She showed more sympathy for her father's efforts to be a good dad, pathetic as they were, and more anger at her mother for leaving them to fend for themselves.
Sarah MacKenzie ran away from home when she was 17 (her junior year of high school), and apparently married Christopher Ragle a year later. They were both drunk for much of their married life. He was later arrested and sent to prison for motor vehicle theft. At some point, Mac finished high school. On the night of her graduation, she was traveling in a car with her friend Eddie. They were both drunk, and were involved in a car accident that killed Eddie. Her uncle USMC Colonel Matthew O'Hara took her to Red Rock Mesa, Arizona, to dry out. Mac later repaid the debt owed to him by making sure he did not get killed during an incident where he and a group of rogue Marines hijacked a van transporting the Declaration of Independence.
After graduating from high school and then at some point earning a bachelor's degree, Sarah MacKenzie joined the Marine Corps and received her commission through Officer Candidate School.
The first known assignment for Mac in the canon was at the grade of first lieutenant at the Headquarters and Service Battalion in Okinawa, Japan, as an administrative officer under John Farrow. It was under his recommendation that Mac was sponsored by the Marine Corps to earn a law degree at Duke University School of Law.
While stationed in Okinawa, Mac engaged in an affair with her superior officer John Farrow. Unbeknownst to Mac, Christopher Ragle followed her to Okinawa and discovered the affair. She was stationed in Bosnia and Herzegovina a year before the events in at the start of the second season, at some point with 2nd Battalion, 9th Marines ("Two Nine") for unknown duration, and given the time-line, was likely part of or associated with the Implementation Force (IFOR), following the 1995 Dayton Agreement.