Cardoe, a Brit, relocated to New York last month.
She recently freelanced for the Department of Health, AstraZeneca and Burson-Marsteller.
She has also worked at Munro & Forster Communications and Galliard Healthcare Communications, and is a former director of comms for the World Cancer Research Fund.
Mermod takes a newly created role as number two to Antonia Betts, managing director of SHI's 30-strong London office.
She is working on AZ, Roche and Boehringer Ingelheim accounts.