Prior to his three-year stint at Octopus, St Claire worked at Microsoft and at O2 UK - taking the title of business PR manager in both organisations.
He will take up his new role at Harvard on 14 February, reporting directly to Paul Bell, CEO of Bell Pottinger.
Bell said he had hired St Claire for his 'strategic depth and his insight into the profound changes that this entire sector is currently undergoing'.
He added: 'Louie and I have discussed his ideas for new initiatives and strategies, but to begin with I would like him to get his feet under the table and familiarise himself with clients.'
The news comes after Lee Brooke left his role as MD of Harvard PR in November to become global comms director for Nokia's new mobile solutions group, for which Good Relations, Harvard's sister agency, handles the UK PR account.
But St Claire will lead a Harvard team bolstered last year by the hires of associate directors Jo Jamieson and Maja Hauke.
Jamieson joined in November from Berkeley PR where she was a director, while Hauke, who joined in March, was previously European PR manager at Sony.
Harvard works with clients including Adobe, Fujitsu, Arqiva and Sennheiser.