Hello,

If I were to have a joint brokerage account with a non-US citizen, that I am not married to, that lives outside of the US--- then how would capital gain tax work? I am the primary account holder. Would I have to pay capital gains tax in his place? Thank you!

If your SSN is on the account, then the IRS would consider the account to be YOUR account, and you would have to report the sales of the stocks on your tax return using Schedule D.