FD, which came to an agreement to release Mainland from the earn-out contract early, is not intending to replace Mainland, who jointly headed the agency's financial services arm with Andrew Waterworth.
Mainland is running the start-up agency on his own, but hopes to recruit up to five staff by the end of the year.
He said he will be focusing on 'classic financial services', with a specialist capability in financial products in the retail, institutional and burgeoning B2B areas.
Mainland sold his original agency, which was set up in 1990 and also called Mainland PR, to FD in December 1999.
Mainland was formerly a director at the now-defunct corporate and financial PR agency Valin Pollen and also previously chairman of the IPR Corporate and Financial Group in 2000.