It is illegal to fire an employee on the basis of gender, race, religion, ethnic background or disability, FindLaw says. It's also illegal to terminate an employee because he notified authorities of the employer's wrongdoing or lodged a complaint against his employer. Employers may not fire workers for performing civic duties such as jury duty or voting.
If an employee is working under an "at will" contract, then either party can terminate the working relationship without any repercussions, the Law Dictionary notes. If the employee signed a contract and the termination was not in accordance with the terms in the contract, she may be able to lodge an unlawful termination complaint.
In order for an employee to successfully sue her employer for wrongful termination, she must collect as much evidence as possible to prove that she was meeting her obligations under the contract. This evidence may include materials such as an outline of events, copies of memos and emails, training videos, performance reviews and the names of other witnesses privy to the events. The employee also has the right to subpoena witnesses, the Law Dictionary notes. Employers have standard policies for how to handle employment and termination, so the employee must prove the manager violated company guidelines by firing her.