Five-strong agency Impact moved into Mantra's London offices this week as a result of the deal, for an undisclosed sum.
In an unusual move, Impact MD Simon Forrest will not join his team at Mantra following his appointment as head of comms at HIT Entertainment.
Mantra co-founder Lawrence Dore said Forrest will continue to work for Mantra on a consultancy basis over the year ahead and is contracted to fulfil an earn-out lasting at least 12 months.
HIT was formerly an Impact client and will continue to retain Mantra going forward.
The acquisition, which was advised on by Madsen Gornall Ashe, sees Impact account director Jane Wilson join Mantra's management team.
Dore said: 'We've been looking to strengthen each area over the last six to seven months and saw dozens of different companies, but this was the obvious fit. It now means we have a full financial capability and a team of up to ten people with financial PR skills to work on quoted plc clients.'
He added they were 'continuing to work with MGA for other opportunities' and would consider further acquisitions over the year ahead if the right deal came up.