It is the first time that Unity, which has a minority stake in the business, has backed a start-up.
The launch of Tin Man follows Sharp’s decision to leave Citizen Relations last October after a period of maternity leave.
Her 16-year career has included co-founding Citizen Brando as part of the Band & Brown Group, where she worked with Unity founders Gerry Hopkinson and Nik Done.
Tin Man will be based in Unity’s central London offices in the short term. However, it is not planned that the two agencies will share clients, with Hopkinson and Done instead offering counsel on growing the business.
Hopkinson, who described the venture as "very much Mandy’s baby", said: "We have always been huge fans of Mandy and are so excited to be working with her on this new venture. We feel the time is right for us to share what we’ve learned in the past few years with someone who has massive talent and experience, and I’m sure there’s plenty she can teach us as well."
Tin Man, which currently has three staff and a network of freelancers, has already been awarded the pan-European social media brief for the Avis and Budget brands. It is believed there was no incumbent on the work and the brands' main consumer and corporate work remains with The Red Consultancy.
Explaining the name of the agency, Sharp said: "In our digital world, we want to make communications human again. That means creating campaigns ‘with heart’ so they really connect with audiences. It also means keeping things simple. Clients, media and bloggers are all people so we don’t do jargon, techie talk or fluff."
Pointing to experience working with clients including BT, British Gas, TK Maxx and Gatwick Airport, she added the aim was to attract major blue chip companies.
"I am ambitious to grow and work with big brands from day one. That’s why I am linking up with Unity, as I know it will help accelerate that growth."