Clift, who announced he was leaving Unilever in February, will lend his experience of global communications to Engine, the owner of agencies including WCRS and Partners Andrews Aldridge, as the business looks to continue its international expansion.
In February, the agency group made its first foray outside of the UK market by launching a shop in the US. The business is fronted by the senior advertising figures Martin Puris and John Bernbach, the son of the legendary DDB co-founder Bill.
Peter Scott, the chairman and joint chief executive of Engine, commented: "Simon's global experience and knowledge speak for themselves. He has a stunningly successful career as one of the innovative and dynamic marketers of his generation."
Clift added: "The changing communications industry is a topic close to my heart, as anyone who knows me is aware, and nobody involved in communications could have missed the exciting development of Engine in recent years. It represents a 21st-century solution within a media marketplace."