Mr Haythorn has more than two decades of experience in the financial services and property services industries.
He is a founding partner of Your Broker and Nectar Mortgages, and was jointly responsible for the launch of Raine & Horne Financial Services and its move into the broker market.
He is also a former chief executive of the Australian Property Institute.
Mr Haythorn said he would focus on helping brokers to improve their performance and profitability.
"I am a great believer that if our broker members' businesses are successful then the aggregation business as a whole grows," he said.
"Serving as a board member over the past year, I have developed solid relationships with all our shareholders, and gained a good understanding of both broker members and the nature of the business."
Chairman Graham Montgomery said Mr Haythorn's appointment comes at a crucial time in the company's growth.
"David's experience is unique in this industry and his track record extends across all aspects of lending, credit, wealth management, valuation and property services, which delivers a new layer of expertise to our business," he said.
Mr O'Brien has left Custom Equity after almost seven years with the aggregator.