The path from public service to private practice actually occurs quite frequently. In fact, many senior partners at law firms have had some experience as a government attorney. Some of the issues that private firms will consider are the transferability of your skills and the current public position that you hold. Many times the skills that a government attorney has acquired from experience on important cases will be the determining factor for firms to consider him or her. For example, an experienced federal prosecutor will have an easier time finding a job in a large law firm than a first-year public defender. That is not to say that only those attorneys in the loftiest government positions can transfer to the private sector, but they will certainly have an advantage in getting into the prestigious private firms.
You must consider the transferability of the skills of your current position and determine if they can be reasonably applied to a private sector job. If your current position is as a district attorney or a federal prosecutor, then the switch to a civil litigation attorney would be a sensible one. The more applicable your current job skills are to your position of consideration, obviously the more favorable your chances are of making a switch. A move such as a trial attorney in the Department of Justice, environment and natural resources division to a position in the environmental defense practice of a firm would be more likely to occur than a move from public defender in the juvenile services program to corporate law.