Cagney, which boasts Brian MacLaurin Associates' eponymous head as non-executive director, is owner of PR agency The Media Foundry, which will work alongside Cubitt on a profile-raising brief.
Cubitt partner Michael Henman heads the account, reporting to Simons.
The agency replaces Holborn PR, which steered the group through its AIM listing in February.
Cubitt will offer financial and corporate PR support, helping to secure coverage in relevant titles.
‘Cagney is keen to grow rapidly in size and shape,' said Henman. ‘It's looking at expanding its portfolio across the marketing services arena with agencies that can operate together or independently.'
As well as The Media Foundry, the group also currently owns ad agency Chick Smith Trott, brand consultancy Brandad and promotional marketing agency Cubo.
Cagney wants to buy ten businesses in the next two years, reaching £100m in market capitalisation in that time.
Simons was CEO of O&M from 1999 to 2002, and set up Simons Palmer Denton Clemmow and Johnson in 1988. Cagney continues to retain Redleaf Communications for project work.