** LEGAL UPDATE **
Many (though not all) non-citizens of the U.S. who have some right to work here must first obtain a plastic identity card called an employment authorization document (EAD). This card must be applied for and received from U.S. Citizenship and Immigration Services (USCIS).
In the past, the next important step was to go in person to an office of the U.S. Social Security Administration (SSA) and apply for a Social Security number (SSN). This number and card is crucial for presenting to employers, so they can withhold tax on your behalf and so that you can file a tax return in April. The SSA also uses your number to track your earnings history and calculate your possible retirement benefits.
Fortunately, the whole process just recently got easier, combining the above two steps into one. Now, when you apply for your EAD and fill out USCIS Form I-765, it has a new box, giving you the option of simultaneously applying for your EAD.
(For details on how to fill out this form, see Filling Out Form I-765, Application for Employment Authorization.)
This change was detailed in a USCIS announcement of October 2, 2017, in which the agency explains its new information-sharing partnership with the SSA. According to USCIS, after you receive your approved EAD from USCIS, you should later receive your Social Security card within another two weeks or so.
Again, however, not everyone who is in the U.S. and wishes to work here is eligible for a work permit. The main categories of people who do qualify include asylees, some students, and people waiting for their adjustment of status (green card) interview or other sorts of USCIS decisions.
Also, some people in the U.S. on temporary visas have the right to work without having to apply for a work permit.
Effective Date: October 2, 2017