I know that both my 19" MTB and my 58cm road bike will fit in the trunk of a 4-door Civic or Corolla, if you fold the seat down, but not neccesarily at the same time.
A fresh -off-the-trail MTB may not be something you want to stuff in the back seat of your car, however, so I understand racking it.Depending on what your parking situation is at work, there are different levels of 'secure enough'

If you work in an office building / complex that has it's own parking area, it will have less exposure to casual by-passers.
Park near or facing the front entrance, where it will be very visible to people coming and going from the building, rather than off in a secluded corner of the lot.
The increased chance of being seen 'in the act.'
If you work at the mall, or park 'on-street,' then bring it inside, or stash it in the trunk.If you leave it on the car, use a security cable, and a large, high visibility padlock; like the kind they sell at Home Depot for locking up lawnmowers and generators and the like. Loop it through the bike frame, wheels, and the rack. Secure it to th tow loop under the bumper.
Nothing is going to be 100% theft-proof to a determined professional, but imho, that threat is pretty rare. Most of the time it's a 'snatch-and-grab' target of opportunity, and a couple of big cables and locks will send that type looking for easier targets.