Before you start

A CURP (Clave Unica de Registro de Poblacion) - your RFC is generated from it
Legal status to work in Mexico (a residency card / work-authorized visa from INM) before you can be put on a Mexican payroll
A Mexican proof of address (comprobante de domicilio), generally under 3-4 months old
Valid official ID (passport, plus your residency card once issued)

Step-by-step

1
Get your CURP first
The RFC is built from your CURP, so this comes first. Foreigners receive a CURP as part of obtaining a residency card from INM; you can then look it up or print it free on the gob.mx CURP service. Without a CURP you cannot self-register an RFC online.
OnlineWho: You (CURP originates from your INM residency process)Issued with your residency card; lookup is instantFree
2
Register for the RFC with SAT
Inscribe yourself in the Registro Federal de Contribuyentes. A salaried person can do this online via the SAT portal using your CURP (well under an hour), or book an appointment (cita) at a SAT office with your CURP, official ID and proof of address. Your employer can alternatively register you through SAT's bulk worker-registration service. Register under the 'sueldos y salarios' (salaries) regime.
OnlineWho: You, or your employer's payroll/HR teamOnline: same day, often under an hour. In person: one appointmentFree
3
Generate your e.firma and Constancia de Situacion Fiscal
After RFC registration, get your e.firma (a digital signature certificate on a USB), which generally requires one in-person SAT appointment with fingerprints. Then download your Constancia de Situacion Fiscal (tax status certificate with your CIF/QR) from the SAT portal or the SAT ID app. HR will ask for this Constancia to set up your payroll correctly.
OnlineWho: You (e.firma enrolment is in person at SAT)e.firma: one appointment; Constancia: instant downloadFree
4
Give your RFC to your employer - they withhold ISR via nomina
Hand HR your RFC and Constancia. From then on your employer calculates and withholds ISR (Impuesto Sobre la Renta) from each paycheck and remits it to SAT, and issues a payroll CFDI (digital invoice) for every payment. You should see ISR withheld on your payslip - you do not pay it separately.
Via employerWho: Your employer (withholding agent)From your first payroll runISR withheld from gross pay (progressive)
5
File an annual return only if you are required to
ISR is progressive - roughly 1.92% on the lowest income up to a top marginal rate of 35%, applied in brackets so only the income within each band is taxed at that band's rate (exact bracket amounts are set yearly by SAT). If you had a single employer who issued payroll CFDIs and you owe no extra tax, you are NOT required to file an annual return (due 30 April). You MUST file if you earned over MXN 400,000 in the year or had two or more employers.
OnlineWho: You (often with an accountant)Annual return window closes 30 April of the following yearFree to file via SAT; accountant fees optional

Documents you’ll need

CURP (Clave Unica de Registro de Poblacion)
Valid official ID - passport and your INM residency card
Comprobante de domicilio (Mexican proof of address, generally under ~4 months old)
RFC + Constancia de Situacion Fiscal (and e.firma) once issued - your employer will request these

Things most newcomers don’t know

Your employer does the heavy lifting - most single-employer salaried workers never file a Mexican tax return.

ISR on salaries is withheld at source through the nomina and remitted by your employer, who also runs a year-end adjustment. If you had one employer with proper payroll CFDIs and owe nothing extra, the annual return is not required - a relief versus the US system many newcomers expect.

Source: SAT - Obligaciones como asalariado

Crossing 183 days (or shifting your centre of vital interests here) makes you a Mexican tax resident on your WORLDWIDE income.

Residency is triggered by more than 183 days in the country, or having your home/centre of vital interests in Mexico. Residents are taxed by Mexico on global income, not just Mexican earnings - so foreign income, rentals or investments can come into scope.

Source: PwC Worldwide Tax Summaries - Mexico

The RFC and its Constancia de Situacion Fiscal get demanded everywhere - sort them before day one.

Payroll onboarding, opening a bank account, signing a lease, and receiving any CFDI invoice all require your RFC, and many ask for the up-to-date Constancia. It is the single most-requested document in Mexican admin life.

Source: SAT - Constancia de Situacion Fiscal

Being on a Mexican payroll does not switch off your home-country taxes - US citizens in particular still file with the IRS.

The US taxes citizens and green-card holders on worldwide income regardless of where they live, so you keep filing a US return (using the Foreign Tax Credit or FEIE to avoid double tax). Other nationalities should check their own rules.

Source: Greenback / Bright!Tax

Common mistakes to avoid

Assuming you never have to file: a second employer in the same year, or income over MXN 400,000, makes the annual return mandatory even though tax was withheld.
Ignoring the 183-day clock - drifting past it without planning can pull your worldwide income into Mexican tax; track your days and talk to a cross-border accountant before you cross it.
Trying to get an RFC before you have a CURP / residency status - the RFC is generated from the CURP, so the immigration steps must come first.
Letting your e.firma expire (4-year validity) or never enrolling it - you will hit a wall when you need to file or change tax status.

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