Collett, who has also previously worked at agencies including Ogilvy PR Worldwide, joined the independent agency this week as an associate director.
She will work across clients including Reckitt Benckiser, BARD and Shire Pharmaceuticals and will also support the board on business development.
Collett began her career as a broadcast medical journalist at ITN but has spent the past 14 years working for agencies.
Most recently at MS&L, which she left in December, Collett promoted Lilly's erectile dysfunction product Cialis and Sanofi-Aventis's new obesity drug Acomplia.
Prior to MS&L, she worked at VB Communications and Ogilvy.
Collett is the first senior hire from outside the agency made at the 22-strong Hertfordshire-based firm since it was established in 1998, said founder and managing director Lesley Scott.
Meanwhile, former hsd director Lisa Penney has resurfaced as an associate director at Nobull Communications. Penney left hsd last year.