The deal, which was concluded in the early hours of Friday morning, affects 50 staff in Jersey and up to 200 in Croydon.
The new combined unit will employ 400 staff servicing 800 clients worldwide, including over half of FTSE 100 companies.
Mourant announced at the start of this year that it was considering a number of options following approaches from several potential buyers.
Although a number of Jersey firms have targeted employee benefit trust (EBT) business, Mourant has been one of the largest players, boosted by the acquisition in 2000 of UK consultancy Mercer.
But the Jersey-based firm says it now wants to focus on its offshore businesses including fund and corporate administration services, private wealth management and legal work.
Although HBOS has headlined the deal as a merger, Mourant say they will not have any further input into the business once the takeover period has been completed.
