The account - the first time Demon Music Group has hired an external agency for corporate PR - was won after a pitch which is understood to have included Le Fevre Communications and one other agency.
Demon - one of the UK's largest music independents - refused to disclose the value of the account.
Mantra's main aim will be to promote the group's back catalogue, including Al Green, Elvis Costello and T-Rex.
Mantra associate director Rebecca Rabin, who will head the three-strong team working on the account, said the main challenge would be to raise awareness of the back catalogue in the music, film and advertising industries.
She will report to Demon head of marketing, Thierry Pannetier.
He said: 'With sales of music in constant decline, record companies and music publishers need to be thinking about new ways to make money from their back catalogues.'
He added that while the company used a range of specialist music PROs to handle media relations within specific musical fields such as country music, Demon felt it needed a larger corporate specialist.