Jupiter head of retail communications Alicia Wyllie said Mainland had been appointed to handle the fund manager's relationship with the financial services press.
Lansons has been kept on to take care of corporate work and provide crisis management advice.
Wyllie said the move was part of Jupiter's strategy to build up its in-house department. She admitted there had been a cut in budget but declined to comment on how significant this had been.
'This is not about saving money,' Wyllie told PRWeek. 'But I am interested in value for money. While Lansons and (chief executive) Tony Langham have tremendous corporate expertise, Mainland was the stronger overall retail team.'
Since joining Jupiter in 2004, Wyllie has made efforts to build up the press office, run until now by Lansons.
Penrose Financial and Quill Communications were among the other companies that pitched for the Jupiter account.