Is it realistic to visit Mykonos and Santorini if staying only 5 days?

Been to both and suggest you do it! They are both very special places. In Mykonos be sure to do a side trip to Delos -- aka "Birthplace of the Gods". Google it and see what you think. If time constraints mean eliminating one or the other then drop Santorini.

Yes, it is possible. We stayed 1 night on Mykonos and 4 on Santorini.

3 days on Mykonos to visit the Chora and go out to it's beautiful beaches to north,2 for Santorini to see the view and Amoudi Harbor. Too crowded, filled with non local businesses and cruise ship hordes,

will be in Athens only for 3 days but leaving ourselves 5 days to
visit Santorini and Mykonos

Don't count the days, count the nights you will be spending in each place. And then look how many full days in between that will leave you with. That will give you a realistic estimate if traveling time and vacationing time are in good balance.

Like if your 3 days in Athens include your arrival and departure day, that means you've got only one full day for experiencing Athens.

And every time you relocate in between, that will cost you at least half a day - getting to the airport/harbor, packing and everything. So from your five days, take half a day (minimum) away for the trip to Santorini, another half to get to Mykonos, and yet another to get back to Athens.

Basically, that means you will have less than two days on each island. If that's worth it for you, go for it.

If time constraints mean eliminating one or the other then drop
Santorini.

I'd recommend just the opposite, especially if you are a first time Greece visitor. Five days isn't enough for peace and quiet (which you won't find on Mykonos anyway), so you might as well go for spectacular.

And another tip: Put those Athens days at the end to be sure you catch your return flight in case of flight or ferry cancelations.

Hello,

Yes, it is possible! Go straight ahead to Santorini. This island is gorgeous... I'd spent most of my days there. Mykonos for 1 day will be fine. It is true, in order to relax you need at least two days. Santorini is much better if you are looking for real vacation and not partying, which I assume you are. Mykonos is less crowded in September, it would make a better choice for vacation then. I've heard the locals say how great Mykonos is in early Autumn, when the crowds leave. You will be able to enjoy it more at another time.

When in Santorini you must try a wine tour... Get some local products both from Santorini and Mykonos. They are superb <3

Sometimes due to the Meltemia (winds that blow heavily across the Aegean), there are ship delays, so I also agree that it is better to spend your days in Athens at the end of your trip. You wouldn't want to miss your flight due to bad weather...