Your balance will show as £GBP. It’s an interesting idea for your balance to show its current value in the the country you are in though.
Transactions made in the USA will show in both $USD and £GBP (pending) at the MasterCard exchange rate at the time of purchase but this will change slightly as the payment settles. The MasterCard rate is generally considered a very good option.
MasterCard is widely accepted in America.
Always make sure you pay in local currency when abroad. So, make sure the card terminal/ ATM shows $USD and decline the offer for it to convert it into £GBP for you.
Although Monzo doesn’t charge you for any ATM withdrawal abroad for the first £200 withdrawn, ATMs in the US will often have their own fees. You will also need to enable the magstrip on your card before you use it in a US ATM - this is in your app settings.
Bringing a spare card with you is always sensible advice though.
Anymore questions? Hope you have a great trip!