Global Radio appointed the agency without a pitch.
Duties include working with its in-house consumer teams on brands such as Capital, Classic FM and Heart.
The radio group previously had a long-term relationship with MacLaurin Media, whose owner Brian MacLaurin announced he was to end work on the account in May.
Global has since been serviced on an interim basis by Insider Communications, a company set up by MacLaurin director Rania Johnson. Insider has begun its handover to Seven Dials and will finish work on the account in November.
Seven Dials was founded by ad agency Beattie McGuinness Bungay in May and has clients including Fortnum & Mason and Aberdeen Asset Management.
Kelner, its chief executive, said: 'This can catapult the agency into a new space.'
He added that he had worked with Global CEO Stephen Miron at The Independent and The Mail on Sunday. 'We know each other of old and they were taken with our fresh approach,' he said.