Guidance for Greencard Holders
Green Card holders are more appropriately known as Lawful Permanent Residents under US immigration law. The "permanent" of permanent residence is significant. Perhaps, nowhere more so than with regard to absences from the United States. Generally, absences from the United States of a year or more by a green card holder are presumed to represent an abandonment of US residence. To learn more about your rights and responsibilities as a Green Card holder, visit the USCIS website page "Rights and Responsibilities of a Green Card Holder (Permanent Resident)."
Extended absences from the US, but with intent to return
Green Card holders who must spend an extended period outside the United States (e.g. to accompany a spouse on an international assignment; to study; to tend to family matters; etc.) may, prior to departing the US, seek permission to maintain their Lawful Permanent Resident status and return to the US beyond the one-year. These "Re-Entry Permits" are generally valid for two years and may be renewed. To apply, an individual must file form I-131 with the USCIS Service Center in Nebraska. A word of caution to those interested in applying for naturalization: Possession of a Re-Entry Permit will not preserve US residence for naturalization purposes; to preserve residence for naturalization purposes, one must file form N-470 (Application to Preserve Residence for Naturalization Purposes). To learn more about naturalization generally, visit the US website.
Unintentional extended absences due to exigent circumstances
As noted above, Green Card holders who fail to obtain permission for an extended absence from the US are likely to be found to have abandoned their Lawful Permanent Resident status. To return to the United States such individuals would normally require a new immigrant visa; however, individuals who have been unable to return to the US due to circumstances beyond their control may seek permission to enter as a Special Immigrant-Returning Resident. Such individuals must still establish eligibility for an immigrant visa, including a new medical examination, but do not need a new immigrant petition.
Lost/Expired Green Card
If you are temporarily outside the United States and have lost your Green Card or your Green Card has expired, don't panic. First, remember the Green Card is not Lawful Permanent Resident Status; it is proof of LPR status. You don't have to go back to the beginning. Second, there is a process for obtaining a Transportation Boarding Foil from the embassy to allow you to board a flight returning to the US. In the case of an expired Green Card, you may not need any special permission at all, depending on the airline.
Giving Up Your Green Card
Not everyone who gains Lawful Permanent Resident status wants to keep it. Whether because of a yearning to return home, an end to business or personal relationships that prompted immigration to the US, or for financial reasons, many Green Card holders are perfectly happy to abandon their Lawful Permanent Resident status. For UK citizens, and citizens of other Visa Waiver Program countries, surrendering a Green Card generally has little effect on one's ability to travel to the US, although those who are subject to inadmissibilities would not be allowed to travel under the VWP, unless a waiver had been obtained in the process of obtaining the Green Card (n.b. those who only had conditional LPR status before abandonment, any waiver obtained for that purpose would no longer apply). Green Card holders who may have established a ground of inadmissibility during their LPR status or subsequent to abandoning it will need to seek a waiver of inadmissibility to visit the US.
N.b. All Lawful Permanent Residents are subject to US-VISIT registration requirements. To learn more, visit the US-VISIT website at http://www.cbp.gov/travel/us-citizens
Additional information from USCIS
You can find additional information about Green Card holder rights and responsibilities, applying for citizenship, and re-entry permits in the USCIS "How To" Guides.