First Time to Greece: Is it manageable to travel from Milos back to U.S. (DC) in one day?

It is not advisable, while Milos does have an airport, not sure if timing of flights link up to catch your flight from Athens, same with ferries. Also, due to winds and weather, both flights and ferries can be disrupted. The sage advise is to usually stay in the Athens area the night before your flight home, plus US bound flights (or connections to those flights) usually leave Athens early morning.

We flew hassle-free from Santorini to Athens to Montreal all in one day, so it can be done. We had about a 3 hour connection in Athens. All will depend upon flight times from Milos to Athens, and your flight from Athens.

It can be done but I have seen many comments on TripAdvisor over the years that the small planes they use on that route have a problem landing on Milos in high winds, make several attempts and return to Athens. This means they cannot take Athens-bound passengers. August can be very windy in the Cyclades.

The Milos airport is small, so large planes can't land there, only the smaller ones. August can be very windy, and if it's too windy the smaller planes are grounded. This happened to me. I had to wait overnight once in Athens for a flight to Milos that was cancelled because of the wind. It's always a good idea to give yourself a last night in Athens so you have a little "time cushion" to make your flight home.

Regardless of the responses, pro and con, there is always a risk. In the dozens of trip we have taken to Greece over the last 50 years we would NEVER not be in Athens the night before a transatlantic flight.

I wouldn't risk it. Ferries and flights from the islands can be cancelled without much notice. You'd probably be fine but the risk is much greater than the hassle of returning to Athens the night before.