Investors pulled $88.1bn (£63.5bn) of assets from Janus Henderson in 2017.
Since the company only had inflows of $77.9bn (£56.1), this means Janus Henderson had net outflows for the year of $10.2bn (£7.3bn).
The company, the product of a merger between the US asset manager Janus and the UK firm Henderson Global Investors, posted revenue growth which was 11 per cent higher, at $1.8bn (£1.29bn).
Janus Henderson said this was driven by higher performance fees earned during the year. It also said it improved its operating margins to 39.6 per cent.
The merger completed in May 2017 and in its results statement the company said the integration of the two businesses was "ahead of schedule".
Andrew Formica, joint chief executive of Janus Henderson, said: "Despite outflows in 2017, we continue to see strong levels of engagement and support from our clients globally and remain encouraged by developing relationships.
"In 2017, we delivered substantial growth in profitability, top-line results and cash flow generation, and we are pleased that we are on track to deliver cost savings greater than originally promised.
"As we look ahead to 2018, we are well-positioned to take advantage of the momentum we are seeing in many areas of our business. Certainly, there is still work to be done; however, we remain committed to providing superior client, shareholder and employee experiences and are excited about the future."
Despite the outflows, Janus Henderson saw its assets under management increase by 16 per cent over the year to $370.8bn (£267.5) because of investment performance.