What time to be at FCO for flight to US at 9:55AM (stopover in Milan) on a Sunday?

The FCO to Milan is a domestic flight so 21/2 hours is enough.
Hopefully you have a layover in Milan the international leg of your flight not too short or long

I have 4 hours layover in Milan. Will TSA check be in MXP (Malpensa) not FCO? Thanks.

There is no TSA in Europe

The first flight is domestic to Milan. So an hour and half to two hours is adequate at FCO. Simply show up, check-in, go through routine security and get to the gate. Your schedule is fine. In Milan you will go through exit immigration, security again and head to the US. That is your international flight. On arrival in the US, you will go through US immigration, followed immediately by customs, and on to your connecting flight home. And security again. First time you will see TSA.

This is a duplicate question. There is one just like it in the Transportation Forum, with much the same answers to the same question.

Note that because you pre-purchased your Leonardo Express ticket, it comes pre-validated and is not good for Leonardo Express trains departing earlier than the time indicated on your ticket voucher (probably 6:50am). Should you arrive at Termini early and board an earlier Leonardo Express train (for example the one departing at 6:35am), you risk being fined. You would need to exchange the ticket to safely take an earlier train, which you probably won't have time to do.

I am assuming this is all one ticket to the US? Flights are from Rome to Milan, Milan to US. You may find you can check in on line but may not get your boarding pass. This will look like an international flight to the airline so you will likely need to show a passport at FCO to get a boarding pass. This is what we found on a recent Alitalia flight connecting with Delta. Our carry on bags did not make the weight cut off so they were checked. You will go through security at FCO. The rules are similar to TSA but no TSA-pre like program. We did not have to take off shoes but you will need your small liquids in a 1 liter bag (one quart worked fine) It went pretty well until they (apparently) saw something that caused them to stop, call back those who had been cleared and re-screen everyone. I was told this was uncommon but lesson here-use the bathroom before getting in line for security, you never know! Your 2.5 hours should give you enough time for this and another cafe!

Thank you Everyone. Looks like my original schedule is OK and will just stick to it as planned.