The ISO 90-Day Exercise Window After Leaving a Job: Miss It and Your Options Become NSOs.

An engineer gives notice at a late-stage startup holding 20,000 vested incentive stock options (ISOs) with a $3 strike and a $13 409A value. Buried in the exit packet is one line: options must be exercised within 90 days of the last day of work. That line is the ISO 90-day exercise window after leaving a job, and it is one of the few tax deadlines that rewrites what kind of option you own. The options do not just quietly expire. Depending on the plan, they either terminate at day 90 or survive as something taxed very differently.

IRC §422(a)(2) conditions ISO treatment on your having been an employee of the granting company at all times from the grant date until 3 months before the date of exercise. Plans almost always write the deadline as 90 days, which is slightly shorter than three months, so the plan document, not the statute, is the date to put on the calendar. Two separate things can happen at that deadline, and they live in different documents. The plan controls whether the option survives: most plans simply terminate unexercised options at day 90, and a terminated option is worth zero regardless of tax law. The statute controls how a surviving option is taxed: if the plan gives you longer and you exercise on day 120, nothing was filed and nothing was elected, but the exercise is taxed as an NSO.

There are two exceptions. If employment ends because of disability (as defined in §22(e)(3)), §422(c)(6) stretches the 3-month window to 12 months. If you die while employed or within 3 months after leaving, Treas. Reg. §1.421-2(c) waives the employment requirement entirely, and your estate or heirs can exercise with full ISO treatment for as long as the plan allows.

Inside the window, an ISO exercise triggers no regular income tax. The spread (fair market value minus strike) is an adjustment for alternative minimum tax purposes under §56(b)(3), the company reports the exercise on Form 3921, and any AMT you pay becomes a credit under §53 that offsets regular tax in later years. Hold the shares 2 years from grant and 1 year from exercise, and everything above the strike is long-term capital gain. Outside the window, the same exercise is an NSO exercise: the spread lands on a W-2 as ordinary wages, picks up Medicare tax of 1.45% plus the 0.9% additional Medicare tax above $200,000 of wages (and 6.2% Social Security if you are still under the $184,500 wage base for 2026), and only growth after the exercise date can ever be capital gain. One route is mostly a prepayment you recover. The other is tax you keep paying no matter how long you hold.

The $27,352 gap understates the difference, because the two numbers are different kinds of tax. The $45,778 of AMT generates a §53 credit that comes back as the AMT and regular tax lines cross in later years, especially in the year the shares are finally sold. The $73,130 never comes back. And the NSO route hides a second surprise: the employer withholds at the flat 22% supplemental rate on the first $1 million of supplemental wages, which is $44,000 on this spread, while the actual income tax is $68,430. That leaves roughly $24,400 due with the return, so an estimated payment in the quarter of exercise saves an underpayment penalty.

A growing number of startups extend post-termination windows to 5 or 10 years, and departing employees routinely read that as keeping their ISOs. They are keeping the options, not the tax treatment. Exercising more than 3 months after leaving fails §422(a)(2) no matter what the option agreement says. The conversion can also happen earlier than people expect: extending the window on an outstanding option is a modification under §424(h), treated as a new grant, and under the §424 regulations an extension offer left open for 30 days or more strips ISO status even if you decline it. My verdict on extensions is still take them. Ten years of optionality on illiquid stock is worth more than the ISO label, but model the exercise as an NSO before assuming the old AMT math applies.

The window forces a decision most people have avoided for years: write a check for the strike plus the AMT on stock you cannot sell, or walk away from the spread. Three numbers drive it. First, your AMT-free headroom. Below a crossover point, tentative minimum tax stays under regular tax and a partial exercise triggers no AMT at all; I walked through that math in how many ISOs you can exercise without AMT. Second, the recovery timeline if you exercise past the crossover, which is what the AMT credit is for. Third, the strike check itself. In the example above, the real risk is not the $45,778 of AMT, most of which comes back; it is the $60,000 of exercise cost that goes to zero if the company does. If you would not buy the stock at $3 with fresh money today, the tax analysis is moot, and letting the options lapse is a defensible answer.