David Ingram replies

If you are a resident alien, you are subject to all US taxes including Medicare and Social Security.

However, if you were there as a temporary (less than 5 years)worker after being transferred from a Canadian branch to a US branch, the COMPANY (not you) can write to the Social Security Administration and exempt you from US Social Security for up to five years.  In this case, the company will pay the CPP to Canada and you will get a T-4 Slip with just CPP on it.

Companies usually transfer their people with an L-1 visa.  It would be unusual for a TN visa holder to qualify but not impossible.  You do have to be working for the same company though and the COMPANY has to apply  for the exemption. 

david ingram
 

We, of course look after these situations by mail - fax - email - courier, etc including the New York City and State returns

david ingram - taxman@centa.com