Medicare is administered by CMS, but you do not enroll with CMS. Enrollment runs through the Social Security Administration, or the Railroad Retirement Board for railroad retirees. That split explains most of the confusion about where to file, and it is why the letter confirming coverage arrives from Social Security rather than from Medicare.
The first question is not how to enroll. It is whether you have to.
Automatic or not: the fork that decides everything else
Somebody already receiving Social Security or Railroad Retirement benefits before 65 is generally enrolled in Part A and Part B automatically, with the card arriving in the post. Everybody else has to apply. Nothing arrives, nothing prompts, and the enrollment window closes on schedule regardless.
Because the age for full Social Security retirement benefits is now later than 65 for everyone reaching it today, the number of people who are automatically enrolled is smaller than the folklore suggests. Assuming a card will turn up is the most expensive assumption in this whole area, and it produces the permanent Part B penalty described below.
Anybody automatically enrolled who is still working with large-employer coverage and wants to delay Part B has to actively decline it, following the instructions that come with the card. Part A generally stays.
Three ways to file, and the one that leaves a paper trail
- Online at ssa.gov. The fastest route for a straightforward Part A and Part B application during the initial window, and it produces a confirmation number worth saving.
- By phone with Social Security, which is the practical route when the situation involves employer coverage or a Special Enrollment Period.
- In person at a Social Security office, by appointment. Slower, but the right choice where forms and employer verification have to travel together.
Enrolling from an employer plan is the case where the paperwork matters. Medicare states that in that situation Part B coverage starts the month after Social Security or the Railroad Retirement Board receives the completed forms, and that an extra form is required showing you had job-based coverage while you or your spouse were working. That is an employer signature, which takes time nobody budgets for. Start it before the month you actually need coverage.
Keep the confirmation, the date, and a copy of the employer form. Penalty disputes are decided on evidence, and the evidence is whatever the applicant kept.
The Special Enrollment Periods that actually exist
The best known runs eight months from the point you or your spouse stop working, or the job-based coverage ends, whichever comes first. Medicare is specific that this clock starts when the work stops, even if you take COBRA or other non-Medicare coverage in the meantime. COBRA does not pause it.
Somebody on Medicare through disability who is covered by a family member’s active employment has a parallel route, with a 100-employee threshold instead of 20. And since 2023 there are additional Special Enrollment Periods for exceptional circumstances, including people who missed a window because of a natural disaster, incarceration, loss of Medicaid, or misinformation from an employer or health plan. Those are granted case by case by Social Security rather than claimed automatically, so the request has to be made and documented.
Two more worth knowing because they are commonly missed. Medicare does not treat retiree coverage or COBRA as active employer coverage for this purpose. And veterans covered through the VA are not exempt: VA coverage does not create a Special Enrollment Period for Part B.
If every window has closed
The General Enrollment Period runs 1 January to 31 March each year, for people who missed their initial window and do not qualify for a Special Enrollment Period. Coverage is not backdated.
The rule here changed for the better and older articles still get it wrong. Coverage now begins the first day of the month after enrollment, rather than waiting until July as it did under the previous rule. Somebody enrolling in January is covered from 1 February. That shortens the gap considerably, but it does not remove the penalty: the late enrollment surcharge described below still attaches.
What a missed window costs, in Medicare’s own figures
| Penalty | How it is calculated | How long it lasts |
|---|---|---|
| Part A, only if you have to buy it | Premium may rise 10 percent | Twice the number of years you did not sign up, then it ends |
| Part B | An extra 10 percent for each full 12-month period you could have signed up. Two years gives 20 percent, which on the 2026 standard premium of $202.90 is $40.58, taking the monthly premium to $243.50. | Generally for as long as you have Part B |
| Part D | An extra 1 percent of the national base beneficiary premium per month without creditable coverage, that being $38.99 in 2026. Medicare’s example: 14 months uncovered gives a 14 percent penalty, or $5.50 a month. | For as long as you have Medicare drug coverage, even if you switch plans |
The Part D figure looks small next to the others and is the one most often shrugged off. It is worth reading as a rate rather than an amount: the base premium is recalculated annually, so the penalty grows with it, and it applies for decades rather than years.
After Part A and Part B are in place
Enrollment in Original Medicare is the government step. Everything after it is a private-market step and happens somewhere else. A drug plan, a supplemental policy, or a Medicare Advantage plan is bought from an insurer, not from Social Security, and none of those enrollments is completed by having filed with SSA.
This is where the enrollment process meets the marketing, and having spent eighteen years on the industry side of consumer marketing and lead generation, I would treat the volume of mail and calls that begins around a 65th birthday as a signal about commissions rather than about deadlines. The plan comparison tool at medicare.gov carries the same plan data every broker uses, priced against the specific drugs somebody takes. State Health Insurance Assistance Program counsellors give the same help free and are not paid on the outcome.
Our companion pieces cover what each part actually pays, the timing decisions at 65, and how Part D works in 2026. No plan or carrier is named on any of them.
Questions readers actually ask
Do I enroll with Medicare or with Social Security?
With Social Security, or the Railroad Retirement Board if you are a railroad retiree. CMS runs the program but SSA takes the applications and makes the eligibility determinations, including whether you have the 40 quarters that make Part A premium-free.
Will I be enrolled automatically?
Only if you are already receiving Social Security or Railroad Retirement benefits before 65. Everybody else has to apply, and nothing arrives to prompt it. Because full retirement age is now later than 65, far fewer people are automatically enrolled than assume they will be.
When does coverage start if I enroll during the General Enrollment Period?
The first day of the month after you enroll. Enrolling in January means coverage from 1 February. This changed from the older rule under which coverage was delayed until July, so any guidance describing a July start is out of date. The late enrollment penalty still applies.
What do I need before I file from an employer plan?
The extra form showing you had job-based coverage while you or your spouse were working, signed by the employer. Coverage starts the month after Social Security receives the completed paperwork, so the employer signature sits on the critical path. Begin it a month or more before you want coverage to start.
Sources
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Every figure in this article traces to a government record or to a named independent, non-commercial research body. We do not cite insurance marketplaces or affiliate comparison sites for data.
- Centers for Medicare & Medicaid Services, Medicare.gov Avoid late enrollment penalties Published 2026-01-01Supports: Late enrollment penalties are added to the monthly premium, are not a one-time late fee, and are usually charged for as long as you have that type of coverage; Part A penalty raises the premium 10 percent and is paid for twice the number of years not signed up; Part B penalty is an extra 10 percent for each full 12-month period, with the 2026 worked example of a 20 percent penalty on the $202.90 standard premium giving $40.58 and a total of $243.50; Part D penalty is 1 percent per month (12 percent a year) after going 63 days or more without creditable drug coverage, calculated on the national base beneficiary premium of $38.99 in 2026, with the worked example of 14 months giving $5.50 a month, added for as long as you have Medicare drug coverage even if you switch plans; no Part D penalty with creditable coverage or Extra Help.
- Centers for Medicare & Medicaid Services, Medicare.gov When can I sign up for Medicare? Published 2026-01-01Supports: Initial eligibility runs from 3 months before the month you turn 65 to 3 months after it; with employer coverage from an employer of 20 or more employees, Part A may be taken at 65 and Part B may wait, with an 8-month Special Enrollment Period after work stops; the 8-month period starts when you stop working even if you choose COBRA or other non-Medicare coverage; where the employer has fewer than 20 employees the job-based coverage might not pay without both Part A and Part B; Part B coverage during the SEP starts the month after SSA or the RRB receives completed forms and an extra form showing job-based coverage is required; sign up for Part B the month before retiring to avoid a gap; a 100-employee threshold applies for coverage through a family member where Medicare is held due to disability or ALS; Medicare eligibility ends Marketplace premium help and keeping it may require repayment at tax time; state Medicaid programs in most cases pay the Part B premium.
- Centers for Medicare & Medicaid Services 2026 Medicare Parts A & B Premiums and Deductibles Published 2025-11-14Supports: Roughly 99 percent of beneficiaries pay no Part A premium having at least 40 quarters of Medicare-covered employment as determined by SSA; 2026 Part A buy-in premiums of $311 with 30 to 39 quarters and $565 with fewer than 30; 2026 Part B standard premium $202.90.
Figures last verified August 30, 2026.