Mutual Trust, set up in 1930s to service the Baillieu family, will merge with the 1925-founded Myer Family Company pending regulatory approval.
Current Myer Family Company chief executive and former Equipsuper boss Danielle Press will leave the merged entity to make way for new chief executive Phil Harkness "after assisting with a smooth transition".
Mr Harkness replaces current Mutual Trust chief executive Tim Hammon, who had already announced his retirement and will continue in a consulting role.
The new chief executive joins from EY, where he was a partner. Before that he spent 11 years as vice president for AT Kearney.
Pending completion, Mr Harkness will be "actively involved in the affairs of both businesses", according to a Mutual Trust statement.
Mutual Trust shareholders will own 65 per cent of the merged firm, whereas Myer Family Company shareholders will own 35 per cent.
The chairman of Mutual Trust Gordon Dickinson said, "Mutual Trust and the Myer Family Company both have focused on helping families with the challenges of managing wealth and their financial affairs across generations.
"As a combined entity, client experience will be enhanced and the financial strength of the combined business will benefit all stakeholders."