Do prospective employees have to provide W-2's or 1099 forms to employers if requested?

I am a California attorney and not eligible to give legal advice in your state. The following is information only. Federal law protects employees from having to disclose their tax returns. The protection includes the return itself as well as indirect disclosure of its contents, which could apply to W-2s or 1099s. A person may not be compelled to disclose amounts of income reported or deductions claimed. Evidently the employer wants to confirm your previous income and perhaps, indirectly, your length of employment. If this is something the employer came up with just for you, then evidently something in your application process caused concern. You may be able to address this concern in a different manner. Your previous employer may provide you with a letter stating your dates of employment and pay rate. This may suffice. Or showing your potential employer a pay stub may do it. You may consider asking the employer what precisely he or she is trying to confirm, and seeing if there is another way to confirm it. Of course, doing this may be interpreted as a challenge and the prospective employer may then decide not to consider you for employment because it thinks you will be trouble.

By the limited amount of information given in your query, it appears that you are offering your services to this company as an independent contractor. Otherwise it would not have been appropriate for you to have performed labor for free (except in rare cases relating to non paid internships). In this situation, any company for whom you perform services may require that you provide them with a Form 1099, an ITIN, or a federal tax id number for their reporting requirements regarding any monies that they pay to you. If this is not the case, please be more specific as to your situation and a more appropriate and relevant response will be possible.