Nathaniel Dalton is succeeding Sean M. Healey as CEO of Affiliated Managers Group.
Mr. Dalton, who was president, chief operating officer and an AMG founder, assumed oversight of day-to-day operations. He retained his role as president but the COO position will not immediately be filled, said spokesman Daniel Gagnier.
Mr. Healey has been diagnosed with amyotrophic lateral sclerosis, or Lou Gehrig's disease, and will require medical treatment. He was named executive chairman.
"I have the utmost confidence in Nate's ability to lead AMG to the next stages of its success," Mr. Healey said in a company news release, noting that Mr. Dalton "has been at my side as my partner since AMG's earliest days and had been involved in every key decision along the way."
Mr. Healey added that he will actively support Mr. Dalton and the rest of the executive team in his role as executive chairman throughout his treatment.
Mr. Dalton said in the release: "As CEO of AMG and with Sean's partnership, it is my honor to lead our outstanding team" and stressed that he stands in "resolute support of Sean as he pursues treatment in his fight against ALS."
Aggregate assets managed by AMG's 33 investment affiliates totaled $831 billion as of March 31.