Wiki has a clear definition for both of those terms:
Mechanization provided human operators with machinery to assist them with the muscular requirements of work. Whereas automation is the use of control systems and information technologies reducing the need for human intervention. In the scope of industrialization, automation is a step beyond mechanization.
In testing, however, I see those two concepts often dangerously mixed: people tend to mechanize testing, thinking they are automating it. As a result, the estimated value of such “automation” is completely wrong: it will not take risks into account, it will not find important regressions, and it will give a tester a false sense of safety and completeness. Why? Because of the way mechanization is created.
Both automation and mechanization have their value in testing. But they are different, and therefore should always be clearly distinguished. Whereas automation is a result of application risk analysis, based on knowledge of the application and understanding of testing needs (and thus finding tools and ways to cover those risks), mechanization goes from the opposite direction: it looks for out of box tools and easiest ways to mechanize some testing or code review procedures, and makes an opportunistic usage of those methods. Mechanization, for instance, is great for evaluating a module/function/piece of code. It is, if you will, a quality control tool, but it does not eliminate a need for quality assurance testing, either manual or automated.
It’s like making a car: each detail of the car was inspected (probably in mechanized, or very standardized way) and has a “QC” stamp on it. But after all details were assembled, do they know if and how the car will drive? If car manufacturers did, they would not have to pay their test drivers. Yet, test drivers are the ones that provide the most valuable input, that is closest to actual consumer’s experience. In software, testers can play a role of both, quality control personnel, and test drivers. Taking away one of those roles, or thinking that test drive can be replaced with “QC” stamp is not the way to optimize testing. And bare existence of mechanical testing should not be the reason to consider application “risk free” or “regression proof”. Otherwise you may end up in a situation where your car does not drive, and you don’t even know.