Smiddy joins from Porter Novelli London where she was responsible for the growth and development of its health and wellness practice. She has also worked at Weber Shandwick and Edelman across the world in the past.
In July Huntsworth Health, which acquired Tonic Life for £3m in 2009, put its founders Scott Clark and Oliver Parsons on gardening leave following the pair giving notice of their decision to quit the agency after failing to reach terms about their future involvement.
Neil Matheson, global CEO of Huntsworth Health, the parent of Tonic Life, said: "We are thrilled to welcome Mary to Tonic as well as our global executive leadership team.
"We continue to expand our global footprint with offices in Doha and Singapore and Mary’s experience in the Middle East and AsiaPAC will be a great asset to Huntsworth Health."
Tierney, who was previously deputy MD, has been promoted to MD of Tonic Life Communications Europe.
She will also lead the Tonic EU Professional Practice with Tracey Carey leading the Tonic EU Consumer Practice.
Matheson added: "With Mary as global CEO, Tamsin leading Tonic Europe and Maryellen Royle leading Tonic US, we have a dynamic leadership team that will ensure Tonic’s position as a leading player in global, regional and domestic multi-channel insight-led health communications."