If you delayed Medicare Part B because you had health coverage from your own job or your spouse’s current job, you usually do not need to wait for the General Enrollment Period.

In most cases, you can apply during an 8-month Special Enrollment Period that starts when the employment ends or the active employer coverage ends, whichever happens first.

The practical path is usually simple:

  1. confirm the date your active employer coverage ended
  2. submit your Part B enrollment as soon as possible
  3. use the right proof of employer coverage so Social Security can process it correctly

That is the part many people wait too long on.

When Can You Apply for Part B After Retirement?

If you had qualifying employer coverage after 65, Social Security says you can sign up for Part B:

  1. while you are still working and covered by the employer plan
  2. within 8 months after the work ends
  3. within 8 months after the active employer coverage ends

Medicare and Social Security both point to the earlier of those end dates.

That is why this timing matters so much:

  1. retirement can start the clock
  2. losing the employer plan can start the clock
  3. COBRA does not extend the Part B Special Enrollment Period

If you need the broader timing rule first, read How Long Do You Have to Sign Up for Medicare After You Retire? and Medicare and COBRA: The Expensive Mistake Thousands of Retirees Make.

Who This Part B Process Is For

This article is mainly for people who already have Medicare Part A and delayed only Part B because they had qualifying job-based coverage.

That usually includes people who stayed on:

  1. their own active employer plan
  2. a spouse’s active employer plan

If you are still sorting out whether your old coverage actually counted, start here first:

  1. What Is Creditable Coverage for Medicare?
  2. Do You Need Medicare at 65 If You’re Still Working? Missouri Rules
  3. Can You Stay on Your Spouse’s Health Insurance Instead of Medicare at 65?

If you delayed both Part A and Part B because of HSA contributions or another working-after-65 setup, your enrollment path can be a little different because you may need to apply for both parts together. If that is your situation, also review Medicare and HSA Contributions After 65: The Tax Trap Many Missouri Workers Miss.

How Do You Apply for Medicare Part B After Retirement?

If you already have Part A, Social Security now lets many people apply online for Part B only during the employer-coverage Special Enrollment Period.

If you are using the paper route, the two forms people usually need are:

  1. CMS-40B for the Part B application
  2. CMS-L564 for proof of employer coverage or recent employment-based coverage

Medicare keeps both forms on its official enrollment forms page, and Social Security uses those same forms when you fax or mail a paper SEP request.

Official pages:

  1. Sign up for Part B only | Social Security
  2. Medicare enrollment forms

The Step-By-Step Process I Would Use

1. Confirm the exact date your active employer coverage ended

Do not guess.

You want the actual end date for the coverage that was based on current employment. That is different from:

  1. the date COBRA ends
  2. the date retiree coverage starts
  3. the date you decide you are finally ready to deal with Medicare

The real Part B question is when the active employer plan or employment ended.

2. Decide when you want Part B to start

Social Security says coverage generally begins the first day of the month after you sign up.

That means waiting can create a gap, especially if your employer coverage is already ending this month.

If you are trying to line up a clean handoff into Medicare, this is usually not paperwork you want to leave until the last minute.

3. Use the online Part B application if your situation fits

If you already have Part A and you are adding Part B during the employer-plan Special Enrollment Period, the online SSA path is often the cleanest option.

That tends to be the simplest route for people who are:

  1. retiring after 65
  2. coming off a spouse’s active employer plan
  3. trying to avoid mailing forms back and forth

4. If you use paper, make sure CMS-40B and CMS-L564 match the same story

This is where preventable delays happen.

Your Part B application and your employer-coverage proof should support the same timeline. If the dates are sloppy, inconsistent, or incomplete, the enrollment can drag out longer than it should.

CMS-L564 is the form that shows Social Security why you qualify for the Part B Special Enrollment Period in the first place.

5. Do not treat COBRA as your Part B strategy

COBRA may keep you insured for a while, but it is not the same thing as protecting your Medicare timing.

If you are retired and thinking, “I have COBRA, so I can deal with Medicare later,” stop there and read Medicare and COBRA: The Expensive Mistake Thousands of Retirees Make.

6. Set up the rest of Medicare before Part B goes live

Part B is usually the trigger point for the rest of the decision-making.

Once Part B is starting, you may also need to line up:

  1. a Medicare Supplement plan
  2. a Medicare Advantage plan
  3. a Part D drug plan

If you wait too long to compare those pieces, you can still create a messy handoff even if the Part B form itself was filed on time.

These guides help with that next step:

  1. How Much Does a Medicare Supplement (Medigap) Plan Cost in 2026?
  2. Medicare Advantage vs Medigap in the Kansas City Area: Which Is Right for You?
  3. What Does Medicare Part D Cover in 2026?

Which Form Does Your Employer Need to Complete?

In the standard paper SEP process, the employer coverage proof is CMS-L564.

That form is used to show:

  1. that you had group health plan coverage tied to current employment
  2. who the employer was
  3. when that employment or coverage ended

Social Security’s internal policy also allows certain written employer statements as proof in some situations, but for most people the practical answer is still the same: start with CMS-L564 unless Social Security tells you to do something different.

What If You Delayed Both Part A and Part B?

That can happen, especially with HSA planning.

Some people delay both parts because they want to keep contributing to an HSA while still working. Others are covered through a spouse’s large employer and decide to hold off on Medicare entirely.

In those cases, do not assume the “Part B only” path is the right one. You may need to apply for Medicare itself rather than just add Part B.

If there is any doubt, use Social Security’s Medicare sign-up pages first and verify which application path fits your case before you submit anything.

Common Mistakes That Slow This Down

These are the ones I would watch for:

  1. waiting until after employer coverage ends to start gathering forms
  2. using the COBRA timeline instead of the active employer coverage timeline
  3. assuming retiree coverage protects the Part B SEP the same way active employer coverage does
  4. sending incomplete or inconsistent employer coverage information
  5. focusing only on Part B and forgetting to line up Medigap, Medicare Advantage, or Part D

A Practical Medicare Checklist for This Transition

If you are close to retirement, I would verify these five things in order:

  1. the exact date your active employer coverage ends
  2. whether you already have Part A
  3. whether you are applying for Part B only or for both Part A and Part B
  4. whether your drug coverage and supplement strategy are ready
  5. whether an HSA issue is still in play

If you want a broader prep tool before you submit anything, use the Medicare Readiness Checklist.

What I Tell Clients Around Kansas City

For people in Blue Springs, Lee’s Summit, Independence, and the broader Kansas City metro, the biggest problem is usually not the form itself.

It is waiting too long because the employer plan, COBRA option, retirement date, and Medicare dates all get blended together.

Once those dates are separated clearly, the Part B application usually gets much easier.

Frequently Asked Questions

How do I apply for Medicare Part B after retirement?

If you delayed Part B because you had active employer coverage after 65, you usually apply during a Special Enrollment Period through Social Security. Many people can apply online for Part B only. If you use paper forms, the standard forms are CMS-40B and CMS-L564.

How long do I have to apply for Medicare Part B after employer coverage ends?

Usually 8 months from the end of the employment or the end of the active employer coverage, whichever happens first.

Can I use COBRA and apply for Medicare later?

COBRA can keep you insured, but it does not extend the Medicare Part B Special Enrollment Period the way many people assume. Waiting for COBRA to end can cause penalties and coverage gaps.

Do I need CMS-L564 to enroll in Part B after retirement?

If you are using the paper Special Enrollment Period process, usually yes. CMS-L564 is the standard proof of employer coverage form used with CMS-40B.

What if I delayed both Part A and Part B?

Then you should not assume the Part B-only application path is correct. You may need to apply for both parts together, especially if you delayed Medicare because you were still working or preserving HSA contributions.