Traveling from Santorini to Athens

1.5 hours should be enough time IF nothing goes wrong. Should your flight back to Athens be delayed or, worst-case scenario, cancelled due to a major mechanical failure, you'll be out of luck. It's NEVER a good idea to plan a return to Athens from an island for the same day as your outbound flight home. The prevailing recommendation is to save all exploration of the mainland for the last days of your time in Greece so as to avoid any issues with getting back in time for that flight. If you can't change your itinerary to return at least the day before your flight home then look for an earlier flight back to the mainland using the skyscanner website. If nothing is available then hope for the Greek gods to smile down upon you.

Most experienced travelers recommend getting back to Athens at least one day before your flight in case of any delays, strikes, bad weather, etc.

You are cutting it way too close. The only alternative is to return to Athens the day before your flight.

Is that enough time to make this transfer to an international flight peacefully?

No.

Logistically possible? As Lee said, IF all goes perfectly and you find favor with the gods.

I would find nothing 'peaceful' about wondering from now until the the day of the flight if it will work.

The general wisdom is 3 hrs for an international flight. You propose to cut that in half, and that is on a best case scenario of your first flight being on time.

Are you willing to bet the cost of a one way walk up ticket to the US of making a 90 minute connection/transfer? Not a bet I would make. Also the next flight to your US destination may not be the same day.

To be redundant, plan your trip so you are in Athens the night before your US bound flight. Hopefully your trip is still in the planning stage.

I can think of no better way to ruin a trip to the Greek islands than to have that nagging worry in the back of my mind for the whole holiday about missing my flight home.

We did just that last year (flying Santorini-Athens, Athens-Canada on the same day). It worked fine for us. But we had closer to 4 hours for the connection. I could have chosen a later flight out of Santorini and had a 90 minute connection, but I thought that was too tight. In retrospect, we would have made it easily with a 90 minute connection. But that is all provided everything is smooth and there are no delays.

We also had the benefit of trip interruption insurance that comes automatically with our credit card, so worst case scenario, we would have had an all-expenses paid extension to our vacation in Athens.