Will be in Rome but flying out of Milan Malpensa airport

Definitely go to Milan the day before and stay over night there. You never know about train disruptions.

Be in Milan the night before
Fast train Roma Termini to Milano Centrale

Day of flight take Malpensa Express- at MPX by 1:00 pm

I would travel to Milan the previous day just in case. Strikes, delays, . . . Who knows. There are direct trains between Rome and Milan offered by both Trenitalia and Italo, taking between 3 and 3.75 hours. Spend the night near MilNonCentrale and take the Malpensa Express to the airport.

Where is the 4.10 flight going? If it is back to the US then starting from Rome is not smart. You need to be at Malpensa around 1 pm to process check-in and immigration check out. It is an hour from Centrale station to the airport. Throw in another hour in case you miss the train. So you would needed to be in Malpensa by 11 am. You need another four hours to get from Rome so you need to board in Termini around 7am. Now I am the cautious type and like to throw in an extra hour or two just in case something happens -- rarely does but need to be safe since not missing the plane in Milan is critical. So I think you should be boarding in Rome around 5 to 6 am. I would not do it but you might like the challenge so you decide.

According to DB Bahn (https://reiseauskunft.bahn.de/bin/query.exe/en) the fast FR (Frecciarossa) trains from Rome to Milan without any changes take about 3 hours. The regionale train from Milano Centrale to Milan Malpensa Aeroporto T1 departs about every 30 minutes and takes about an hour. If you leave no later than 07:50 from Roma Termini, you could conceivably get to the airport by 13:00, the required 3 hours before your 16:10 flight, but if anything goes wrong you'll miss that flight.

The details above are based on a DB Bahn search I did for train travel on 17 May. You'll need to use the DB Bahn link above for your actual travel day(s). And note that when you find a good time for you, you'll need to make your reservations with Trenitalia, the company that actually provides the train service. The link below might be helpful, or at least get you started exploring. It's in English. I didn’t work with it much, but there should be some significant price reductions in purchasing your reservations online and printing your tickets at home sooner rather than later. Frecciarossa from Rome to Milan, an unbeatable offer:
https://www.trenitalia.com/it/le_frecce/viaggia_da_roma_amilanoconlefrecce.html

I hate rushing, so I'd choose to take the fast train from Roma Termini to Milano Centrale the day before the flight. Once I went on to an airport hotel to spend the night. Once I stayed near the Milan train station and took the regional train to the airport the next day.

The latter was much better than the former. I spent the night in Milan near Milano Centrale, had a great leisurely breakfast at my hotel (Hotel Berna: https://hotelberna.com/en/hotel/ ) the next morning, walked the couple of blocks to Milano Centrale and took the regional train to the airport. Malpensa is large and spread out. Be prepared for a long walk to your gate.

Bonus link to info on and pictures of the Frecciarossa trains: https://www.seat61.com/trains-and-routes/frecciarossa.htm

So if those trains are run by another company what's the point of searching on a secondary source site like DB?

The trains from Milano Cadorna station to Malpensa are 20 minutes quicker than those from Milano Centrale and the area around Cadorna is nicer.

I greatly appreciate all the advice and the precise tips by Frank, Lo from Tucson, and Dario.