Carlyle chief executive Kewsong Lee is leaving the private-equity firm, as it struggles to expand and its shares lag.

Carlyle said late Sunday after The Wall Street Journal inquired about the matter that Lee will step down as CEO immediately and will leave the firm when his five-year employment agreement ends at the end of this year. William Conway, a co-founder and former co-CEO of the firm, will serve as interim chief executive until a permanent successor can be found.