Do L-1 intracompany transfer visa holders pay U.S. Social Security and Medicare taxes?

Yes, in most cases. L-1 transferees are not exempt individuals, so FICA (Social Security and Medicare) is withheld from your U.S. wages from day one, just like for any U.S. employee. The main exception is if your home country has a totalization agreement with the U.S. and your employer keeps you on the home-country social security system with a certificate of coverage; then you may be exempt from U.S. FICA for a limited assignment period. Countries like the U.K., Canada, and Germany have such agreements, but India does not, so Indian L-1 workers generally pay full U.S. FICA. The Social Security Administration lists current agreements at ssa.gov.