Mr Howell will be replaced by finance director Paul Stockton, the firm revealed in an announcement to the Stock Exchange.
Mr Stockton joined the firm in 2008.
Mark Nicholls, chairman of Rathbones, said: “Under Philip’s leadership, Rathbones has firmly established itself as the leading UK wealth manager and, on behalf of the board, I would like to thank him for the strong direction, unfailing commitment and dedication he has provided to Rathbones during his tenure.
“The board recognises the importance of careful succession planning at Rathbones and, having worked with Paul for many years, we are delighted with his promotion to chief executive officer.
“Paul has built up considerable experience and a deep knowledge of the firm, its values and culture and I wish him every success as he takes on his new responsibilities.”
Philip Howell, chief executive of Rathbones, said: “It has been my privilege to lead and work alongside the many talented professionals and dedicated support staff who compose Rathbones.
“I am very grateful for their considerable contribution to our success over the past five years.
“I look forward to working closely with Paul through the transition and I am confident that he will lead the firm to continued success.”
Paul Stockton, group finance director/managing director Rathbone Investment Management, said: “The opportunity to lead such a prominent and well-respected business is truly exciting.
“Our clients and stakeholders place considerable trust in our ability to deliver them a quality service and reliable investment outcomes.
“I very much look forward to continuing to work with the board, our executive team and my colleagues, to build on our position as a leading UK wealth manager.”