Ellen MacKenzie, nick named `Mac', is the cop in charge of the unit, which means she overseas all the operations performed by the unit.
She went from a B.A. Honors in Criminology to being in charge of the unit in only a few years.
She was brought up in Sydney, right on the harbour but later moved to Melbourne. she has had a working relationship with Peter Church and the are particularly close, but since his relationship with Collette and her love affair with Bill Hollister, the pair have since drifted apart.
Ellen was originally going to be one of those really tough `boy-boys', but now recently she has been embracing a more feminine side to her character. She is close friends with all the unit, particularly Pete and Angie. But since Stones shooting and the I.A. (Internal Affairs Investigation) Investigation, into the shooting of Angie's informant, they too, have also drifted apart. Mac is also Close friends with Oscar and Danni.
Earlier in the season saw her involved in a hold up at a service Station, where she was stabbed with an HIV+ syringe and had to wait six weeks to find out whether she'd contracted the virus. During that time she received great support from Pete and after six long weeks was cleared of the HIV virus.
Despite the pressures of working together MacKenzie later managed to develop a blossoming romance with Bill Hollister despite the obstacles, such as work and Bill's troubled teenage daughter, Sara (Beyond Redemption). However, things ended in tragedy for the troubled lovers when Bill was killed in a car bomb explosion.
Ellen has just recently learned she is adopted and after firstly, finding herself bitterly disappointed in the results of her search for her long lost mother (Closure) was finally reunited with her real birth mother, photographer Eve Reisner (True Colours).
Toward's the end of last year she received an invitation to apply for head of Homicide meaning she would have to leave the Undercover Unit. The situation has resulted in her and Church being brought closure together again (Reunion).
Things unfortunately when very wrong when Ellen `Mac' MacKenzie when she found herself the victim of a grifter, who wound up having her drugged, on the eve of her interview and to make matters worse any intention of her rekindling an old flame with Church were ruin after discovering he was the father of Danni's baby (An Anomynous Guy).
After these set back MacKenzie set about trying to pick up the pieces and get her life back on course, leading her to become involved in a romantic relationship with one of the newcomer, Det. Nick Jardine of Homicide, however just as things were starting to look up for Mac she was hit with yet another bombshell when she found herself framed for the murder of John Murray, the Grifter who had her drugged and was about to confess all to her, about the conspiracy by the head of current Head of Homicide to have her drugged in order to sabatage her chances of obtaining the job as Head of Homicide, when he wound up dead under a car after being pushed (What's Love Got to Do With It?), and found herself on suspension and is now faced with the prospect of being charged.
