The pre-tax figure rose from £2.5m last year to £3.5m this year.
Included in the profit figure for the six months ending 30 June was an estimated income of £2.3m for Secure Trust Bank PLC (STB) being an associated undertaking.
Financial highlights included an increase in net assets from £234m in the first half of 2017 to £235m this year and an increased interim dividend from 14p to 15p.
Operational highlights included a 25% increase in customer loans from £879m to £1,097m, a 25% boost in customer deposits and a management team joining to set up a specialist secured lending business.
Assets under management also rose 7% from £1,001m in the first half of 2017, to £1069m this year.
Sir Henry Angest, chairman and chief executive of Arbuthnot, said: “The group has continued to develop.
“The asset based lending business has commenced trading ahead of schedule.
“We have also reached an agreement with a management team to establish a specialist secured lending business.
“Given the continued growth and diversification of the group, I believe our long term prospects are encouraging.”