Kleiner will be responsible for working with social media experts in each of Havas Media UK agencies, including MPG, Media Contact, Arena BLM Quantum, AIS and Cake.
Kleiner joins from IPG-owned Momentum where he was global social media strategist and developed global media strategies for global clients such as Coca Cola, Sony and Nokia.
Havas Media UK chief executive Mark Craze said: "Social media is a complex and massive opportunity for our clients to get a competitive advantage.
"It is evolving at a breakneck speed and changing on a daily basis, which makes it both scary and exciting. Neil will ensure that we remain at the cutting edge of the market in delivering outstanding strategic and implementational advice to our clients."
In February Havas reported it had suffered a 14.3% year-on-year decline in organic growth in the UK in the fourth quarter of 2009, as the French marketing group said its operations were still taking a hit from the "steep reduction in advertising spend in the UK".