When a person comes to the United States on a K1 fiancé visa, they are not allowed to work in the country until they apply for their work permit. In order to apply for a work permit, the foreign fiancé will have to apply for adjustment of status, which is the process of getting your green card within the United States.
When a foreign fiancé arrives in the United States on a K1 visa, they will have 90 days to get married. Once married, the foreign fiancé can apply for his/her green card through a process called adjustment of status. When applying for adjustment, you may also apply for your work permit and travel document. Once your work authorization is approved, and you have received your EAD card, you can legally work in the U.S.