I am assuming the spouse of the US citizen does not have a green card. The short answer is both the US citizen/petitioner and the Mexican citizen/beneficiary must live in the US.Now for the long answer, the process for getting a green card is long and complicated. However, as the spouse of a US citizen you are in luck! You will qualify for the family based spousal green card. The highest priority for a green card is given to the spouse of a U.S. citizen. This category of immigrant is not required to get a visa before applying for their green card. First the US Citizen/petitioner must file an I-130 - Immigrant Petition for Alien Relative. This form tells the US immigration services that a US citizen wants to bring their immigrant spouse to the US. You can file this form while living in Mexico. There is no requirement to be living in the US when filing an I-130. When living outside the US, you should mail your I-130 to the USCIS Chicago Processing Center (Attn: I-130, 131 South Dearborn-3rd Floor Chicago, IL 60603-5517)Now to answer your questions. Where you will run into issues living outside of the US is with the I-864 - The Affidavit of Support. According to the USCIS, this document shows that the petitioner accepts financial responsibility of an immigrant who is coming to the US. Basically, the US citizen who signs this form becomes the sponsor. In order to be a sponsor for the I-864, you must have a US domicile and the I-864 must be submitted by an I-130 petitioner.The good new, however, it that the UCSIS sees it sufficient for the US citizen petitioner to show evidence that they will return to the US once the I-130 is approved and the immigrant is eligible for an immigrant visa.Once you are in the US and ready to submit your I-864 and any other documents you can use a service like SimpleCitizen | The Digital Immigration and Visa Solution to guide you through the process.