Hornby, who has returned to the agency after seven months' maternity leave, has a brief to help build new business and more clearly define its offering to client prospects.
She takes over a role which has been shared by Minnie Moll, the marketing director, and Nick Howarth, the managing director.
With Hornby in place, Moll will take responsibility for some of HHCL's clients including the recently won Bhs and COI Communications.
Hornby began her career as a graduate trainee in the Harrod's marketing department. Her first advertising job was at the Mojo agency in Sydney, where she worked on the Qantas account.
She returned to the UK after two years to join CDP where her responsibilities included Honda, Canon and the Early Learning Centre. Since arriving at HHCL in 1999, she has looked after Iceland, Texaco, Robinsons soft drinks and some COI business.
Her appointment follows a concerted effort by HHCL to get itself back on course after a string of account losses in 2002. Now under WPP's wing, the agency's record has improved significantly and includes the capture of Sky TV, its biggest-ever win.
Hornby said: "We've been through a period of enormous change and, in winning new business and getting back on our feet, we've not stopped to consider what we should be saying about ourselves. We had a clearly defined position in the 90s. Now it needs reframing."
Howarth said: "I'm thrilled that we've got someone of Clare's talent and energy in this role."