Tinopolis managing director Arwel Rees said the move was a result of a reversal into cash shell Acquisitor before the company's listing in February.
Acquisitor had retained Bishopsgate, 'so we inherited it', Rees said.
He added that Mantra's financial comms brief would be to draw investor and broker attention by highlighting earnings growth potential.
Tinopolis was recently named preferred bidder for the BBC's digital curriculum programme, which will roll out programmes for schools.
Tinopolis shows include Wedi 7, S4C's nightly magazine programme.