Onboarding in Mexico goes wrong most often on a timing detail, and a simple scenario shows why. A company hires someone who starts on Monday and files the IMSS social-security registration (the alta) later that week, treating the legal five-business-day window as a grace period. On Wednesday the new hire slips on a wet floor. Because the worker was not yet registered, the employer, not IMSS, pays 100 percent of the medical costs, plus a penalty and back contributions. That is why, in practice, employers register a worker the business day before the start date. Mexican onboarding is a chain of registrations, each with its own authority and its own deadline, and the important part is knowing which ones must be done before day one. Here is the chain.
Before the first hire: the employer registrations
Mexico has several government bodies that each need to register the employer, and no one can be onboarded until those registrations are in place.
The employer needs an RFC with the tax authority (SAT) and an e.firma, the digital certificate used to stamp payroll receipts; foreign companies routinely get stuck here because the e.firma has to be obtained in person. The employer also needs an IMSS employer registration (registro patronal) with an NRP number, plus IDSE access to file worker movements electronically. INFONAVIT, the housing fund, comes automatically with the IMSS registration, so that is one registration the employer does not arrange separately.
Then there is a step companies from single-country backgrounds miss: state payroll tax (ISN) is registered separately in every state where the employer has staff, because Mexico has 32 independent state tax authorities. Rates differ by state (for 2026, Mexico City is 4.0%, most states are around 3.0%, Baja California is 4.25%). At IMSS registration the employer also self-classifies its risk class (I to V), which sets its workplace-injury premium; under-classifying to save money is the kind of thing that gets caught on audit as back-payments plus a penalty.
Collect the worker's identifiers
Before the first payroll the employer needs the worker's CURP (the population registry ID), their RFC (tax ID), and their NSS (social security number). A few things are worth flagging. The generic placeholder RFC (XAXX010101000) is not valid for an active employee's payroll receipt, so if a worker lacks a real RFC the employer has to request their inscription with SAT. For a first-time worker, IMSS generates a brand-new NSS at the alta; for everyone else, the existing number should be verified rather than duplicated. The employer also collects a CLABE for salary deposit, an official ID (INE), and the documents that go in the employee file (expediente), and must give SAT the worker's email and phone.
Register with IMSS before the first day
This is the deadline that matters most. The Social Security Law (Ley del Seguro Social, Art. 15) requires the employer to register the worker within five business days of the hire date. But social security coverage backdates to the declared start date, and an unregistered worker who has an accident is an uninsured worker, so the strong practice is an alta previa: filing the business day before the start date. IDSE accepts pre-registration up to one business day ahead. There is no deadline extension for these notices, so if day five lands on a Friday before a holiday, it is still day five.
At the alta the employer sets the SBC, the salario base de cotización, the daily integrated salary that all IMSS contributions are calculated on. This is a Mexico-specific concept worth understanding: the SBC is not just base pay, it bundles in benefits such as the aguinaldo and vacation premium through an integration factor, and it has a ceiling of 25 UMA. Set it wrong at enrollment and the employer faces an assessment for the difference plus a surcharge of 40 to 100 percent. The worker's Afore (pension fund) is routed automatically through IMSS; the employer does not deal with Afores directly.
Put the contract in writing
Mexico requires a written employment contract with all the mandatory elements listed in the Federal Labor Law (Ley Federal del Trabajo, Art. 25): the parties with CURP and RFC, the type of relationship, the duties, the workplace, the hours, the salary and how and when it is paid, rest days, vacation, and training. If there is no written contract, the worker does not lose any rights; the fault is the employer's, and the relationship is presumed to be on the default terms.
That default matters: under Art. 35, if nothing else is stated, the relationship is indefinite (por tiempo indeterminado). A probationary or fixed-term arrangement has to be written down. A probationary period (Art. 39-A) can last 30 days for a general role, or up to 180 days for management, executive, or specialized-technical roles, and it only applies to indefinite contracts or fixed terms longer than 180 days. An initial-training period (Art. 39-B) lasts up to three months, or up to six for management. The worker gets full pay and benefits throughout either one, and if the probation is not in writing it is void and the worker is indefinite from day one.
Set up payroll and the statutory benefits
Several things have to be configured before the first payroll is correct.
Income tax (ISR) is withheld each pay period as a provisional payment. Lower earners also receive the employment subsidy (subsidio al empleo), so the calculation must include it. Every payment needs a CFDI de nómina, the electronic payroll receipt stamped through SAT; without a valid stamped receipt, the salary is not tax-deductible for the company, so this is not just a formality.
Then there are the statutory benefits every Mexican employee is owed, set up at onboarding even though they pay out later:
The aguinaldo, a year-end bonus of at least 15 days' salary, paid by December 20 (Art. 87).
Vacation and the vacation premium: since the "vacaciones dignas" reform, the first-year minimum is 12 days, and the vacation premium (prima vacacional) is 25% of the vacation pay (Art. 76 and 80).
Profit sharing (PTU) eligibility, flagged at onboarding, noting that temporary workers with fewer than 60 days in the year are excluded.
State payroll tax at the rate for the worker's state, remitted monthly.
One newer obligation belongs on the onboarding checklist: as of January 2026, employers must deliver training to prevent violence against women, and new hires should receive it within their first 30 days, with a signed acknowledgment kept on file. It fits alongside the psychosocial-risk and anti-harassment onboarding Mexican employers already provide.
For reference, the 2026 daily minimum wage is MXN$315.04 in the general zone and MXN$440.87 in the Northern Border Zone, and the UMA (the unit that caps and exemptions are expressed in) is MXN$117.31 a day.
Keep the file, because the burden of proof is on the employer
One habit separates smooth Mexican payroll operations from painful ones: keeping the employee file complete from day one. If a dispute reaches the labor authority and the employer cannot produce the documents, the worker's version of the facts is presumed true. Employers should keep contracts, receipts, and IMSS records for at least five years. This is not bureaucracy for its own sake; the file is the evidence that protects the company.
Where automation earns its place
What makes Mexican onboarding easy to get wrong is that the registrations are split across authorities, states, and individual workers, and the SBC has to be calculated per person rather than copied. Done by hand across several states, the errors are rarely dramatic; they are quiet, and they show up on an IMSS or SAT audit two years later as back-contributions and surcharges. Flux handles the mechanical parts: we file the IMSS alta on time, calculate each worker's SBC integration factor, apply the current minimum wage, UMA, and state ISN rate for the correct work location, and stamp the payroll receipts. That leaves the payroll administrator free for what a person should do in someone's first week: making sure the new hire is set up to do their job, rather than racing a five-day clock.
Niko Nurmentaus
Product Lead
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