(covers information from several alternate timelines)
Prior to serving aboard Enterprise, McKenzie conducted a six-week tour on Jupiter Station, where the periodic failure of the artificial gravity led her to gain a proficiency at operating in gravity boots. (ENT: "Anomaly")
McKenzie accompanied Captain Archer and Lieutenant Reed to investigate a derelict alien vessel which the Enterprise crew had encountered in the Orassin distortion field. The next year, McKenzie's commanding officer, Major Hayes, recommended that McKenzie be given command of the team, after he had been mortally wounded on a boarding mission to a Xindi-Reptilian warship. (ENT: "Anomaly", "Countdown")
In an alternate timeline in which Enterprise was thrown back in time from 2154 to 2037, McKenzie developed a romantic relationship with Enterprise's helmsman, Travis Mayweather, and the two eventually married. This timeline was eventually negated when, with the help of the descendants of their temporally-displaced counterparts, the Enterprise crew managed to avoid the accident which had thrown them back in time in the first place. (ENT: "E²")
In the non-canon novel Last Full Measure her first name is listed as Fiona, although her on-screen uniform ID patch carries the initial "J".