The appointment of Garratt, who joins this week and reports to head of healthcare Sally Bradford, follows a reorganisation at the agency that saw the splitting of the healthcare team into global and UK sections.
Garratt's focus will be on bringing in new clients as well as heading ethical work with Sanofi-Aventis, the healthcare group's main UK client, according to Bradford. This includes promotion of its cancer treatment, Taxotere.
During her time at Avenue HKM she also worked with Sanofi-Aventis, promoting brands including heart attack and stroke treatment Plavix.
Garratt said: 'I'd been at Avenue for a long time and this came up and appealed to me because MS&L doesn't have a big UK PR team - but it is something they are looking to increase.'
Garratt joins a director team that includes Annabel Fiddian-Green and Guy Nasse, who both have the title of specialist director and work across the UK and global teams.
A fourth director, who will also report to Bradford and handle day-to-day running of the global healthcare business, is due to be appointed in July.
Prior to Avenue HKM, Garratt spent five years at Blackwell Healthcare and Current Drugs.