As Murray Rothbard States and via Mises.org
In truth, there is only one way to regard a minimum wage law: it is compulsory unemployment, period. The law says: it is illegal, and therefore criminal, for anyone to hire anyone else below the level of X dollars an hour. This means, plainly and simply, that a large number of free and voluntary wage contracts are now outlawed and hence that there will be a large amount of unemployment. Remember that the minimum wage law provides no jobs; it only outlaws them; and outlawed jobs are the inevitable result.
All demand curves are falling, and the demand for hiring labor is no exception. Hence, laws that prohibit employment at any wage that is relevant to the market (a minimum wage of 10 cents an hour would have little or no impact) must result in outlawing employment and hence causing unemployment.
If the minimum wage is, in short, raised from $3.35 to $4.55 an hour, the consequence is to disemploy, permanently, those who would have been hired at rates in between these two rates. Since the demand curve for any sort of labor (as for any factor of production) is set by the perceived marginal productivity of that labor, this means that the people who will be disemployed and devastated by this prohibition will be precisely the "marginal" (lowest wage) workers, e.g. blacks and teenagers, the very workers whom the advocates of the minimum wage are claiming to foster and protect.
The advocates of the minimum wage and its periodic boosting reply that all this is scare talk and that minimum wage rates do not and never have caused any unemployment. The proper riposte is to raise them one better; all right, if the minimum wage is such a wonderful anti-poverty measure, and can have no unemployment-raising effects, why are you such pikers? Why you are helping the working poor by such piddling amounts? Why stop at $4.55 an hour? Why not $10 an hour? $100? $1,000?
It is obvious that the minimum wage advocates do not pursue their own logic, because if they push it to such heights, virtually the entire labor force will be disemployed. In short, you can have as much unemployment as you want, simply by pushing the legally minimum wage high enough.
Rothbard says it far more eloquently than I ever could.