She reports to UK healthcare MD Catherine Keddie, joining a 30-strong division whose revenues grew by a third in 2016.
Tierney joined the WPP agency last month.
Tierney had arrived at Tonic Life in late 2013 initially as deputy MD under MD Stephanie Bunten, and became MD a year later, after founders Scott Clark and Oliver Parsons were put on gardening leave by new parent company Huntsworth.
Prior to Tonic Life she worked for Just Health PR and before that H+K Strategies.
Keddie said: "Tamsin is a fantastic hire and combines senior experience with bags of energy, and shares our vision to build a healthcare business at the cutting edge of integrated pharmaceutical marketing."
Tonic Life is led from the US by Philadelphia-based global CEO Maryellen Royle. Its London office is situated in the same premises as other Huntsworth agencies.