The purchase, formalised on 5 January, will include a loan book of £33m and deposits of £9m. Three members of the Rathbone private banking team will join Butterfield: senior manager Keith Brown, manager Simon White and Francesca Sokoli, the client relationship officer.
Antonia Blake, head of marketing and business at Butterfield, said: "The best benefit to our charity clients is that we will have three more private banking relationship managers.
"Our aim is to provide a very high level of service, with personal contact, so that our clients can speak to people with long-term experience and knowledge of the sector."
Butterfield acquired Leopold Joseph Holdings in April 2004. Charity money accounts for a third of the bank's investment management business.