The deal, for an undisclosed sum, sees Lord North Street become a wholly owned entity of SandAire, although both firms will continue to trade under their existing brands.
SandAire founder Alex Scott will become executive chairman, while William Drake and Adam Wethered, co-founders of Lord North Street, will be vice-chairmen.
The new firm has also appointed Lansdowne Partners’ Alexandra Altinger as chief executive.
Currently both firms advise wealthy families with at least £30m in assets, as well as endowments for Oxford and Cambridge colleges.
Mr Scott said the deal would “free up” the firm’s founders to have more contact with its clients.
The deal is subject to FCA approval.
In February Deutsche Bank decided to sell Tilney, a national wealth management business aimed at high-net-worth clients, to private equity firm Permira for an undisclosed sum.
The deal was agreed with a view to merging Tilney with London-based advisory firm Bestinvest.
Bellpenny also completed its 16th and 17th acquisitions earlier this month of Essex-based Personal and Corporate Financial Solutions and Woking-based MDM Associates.