For both historical and financial reasons, American Express acceptance in France is quite low.
Visa and MasterCard were associated from the start with the CB program, so anywhere they take cards they will take Visa and MasterCard, it's a standard part of the contract a merchant would get from their bank for card acceptance.
On the other hand, American Express acceptance often needs to be requested separately, and the fees are often much higher, so merchants often don't bother.
American Express cards will be accepted in most upscale hotels, restaurants and shops (mostly those that cater to high-end tourists), but other than that, it's pretty much a lottery.
American Express have been working hard to expand acceptance, so you can have some nice surprises.
They are actually accepted in stores like Carrefour (though not the very small ones) or Monoprix.
They have a site where you can look up merchants which will accept AmEx: https://www.americanexpress.com/fr-FR/maps
As a side note, even if the stores accepts contactless payments and American Express, it's nearly impossible to find a place where you can use American Express with contactless or Apple Pay, the terminals are usually not configured correctly for this.
You will most likely need to have a Visa or MasterCard card, or use cash, in most places.
Note also that ATMs that allow you to make withdrawals (cash advances) on American Express are quite rare as well.
travel.stackexchange.com/users/30703/jcaron
I am in France for business and personal... Can only use my AMEX when I am in a very obvious "business" situation.
Hotels fine (maybe not cheap ones).
Restaurants near business districts usually OK but still not 100%.
Random restaurant maybe 25% if that.
Shops are hit and miss.
A high-end store might accept AMEX 60-70% but a mom and pop might be 10% - again usually only if they are in a business district or near a hotel with business travelers.
In Paris I can use my AMEX 80% of the time in the financial district and about 20% of the time in other parts.
So the answer is if you are only bringing an AMEX you better bring tons of cash.
travel.stackexchange.com/users/36071/blankip
It's well worth paying the 'ridiculous' fees for a Mastercard or Visa card.
Or at least searching for a reasonable one; they exist.
American Express is basically only accepted in tourist traps, places where you'd expect some visiting American to wander into with their business expenses.
Not exactly the best way to spend a vacation in France, and also very hard to find outside of Paris.
travel.stackexchange.com/users/None/
I'm French and I have both.
There are no problems using Visa anywhere.
Because of transaction fees, some shops may not accept American Express cards.
In big cities you won't have any problem with your American Express card but in a small village I am not so sure.
Contactless payment is becoming more popular but having a Visa or Mastercard would be preferable.
Have a nice trip.
You can try asking (before shopping): Prenez-vous les AMEX ou seulement visacard?
(do you accept AMEX or only Visa cards?)
travel.stackexchange.com/users/98313/a french guy
How much can I count on this being accepted?
Some places would accept them however certainly nowhere as widely accepted as in the USA.
I used my AMEX for dining when I visited.
Always prudent to have some cash, especially for smaller establishments and hole-in-the-wall locations.
If I got a VISA or MasterCard, would I have better acceptance?
Absolutely TL;DR I am extremely surprised you indicate you could only find American Express cards without foreign transaction fees because there must be at least five times as many Visa/MasterCard cards without transaction fees as American Express cards.
travel.stackexchange.com/users/56513/user 56513
I'm going to be in France — mainly cities, especially Paris and Brest — for about a month this summer and the only credit card I have without ridiculous foreign fees is AMEX. How much can I count on this being accepted?   If I got a VISA or MasterCard, would I have better acceptance?
Is American Express widely accepted in France?
travel.stackexchange.com
<france><payment-cards>
travel.stackexchange.com/questions/139427
travel.stackexchange.com/users/2664/Joshua Frank
Historically, American Express is not widely accepted in France due to the transaction fees. Most upscale establishments will accept AMEX but this cannot be assumed in other places. As well as cash, VISA and MasterCard are more viable alternatives due to their inclusion from the start of the CB program. American Express is often not accepted in France, therefore it is worth carrying Visa, MasterCard and/or cash as well.
American Express is often not accepted in France because of fees, it's worth carrying cash even if more places are accepting American Express. Visa and MasterCard are more commonly accepted, so you should have them or carry cash if you're planning to use American Express.