American International Group Inc. on Wednesday said it has named Thomas Lillelund as CEO of AIG Europe S.A., one of the company’s new legal entities to handle its European business post-Brexit.

AIG Europe S.A., based in Luxembourg, is a new legal entity for AIG’s business in the European Economic Area and Switzerland, established to provide business continuity for AIG’s European operations after the United Kingdom leaves the European Union, the insurer said in a statement.

Mr. Lillelund, who will be responsible for continental Europe and Ireland, will report to Chris Townsend, CEO of international general insurance, the statement said, adding that the appointment is subject to the required regulatory approval.

Mr. Lillelund joins AIG from Aspen Re, a unit of Bermuda-based Aspen Insurance Holdings Ltd., where he had been CEO.

AIG Europe S.A. is one of two new AIG units, along with American International Group UK Ltd. in London. AIG will transfer existing business to the new U.K. and European entities and merge its existing division, AIG Europe Ltd., into the new European company. Both new companies will begin writing business starting Dec. 1, 2018, according to the statement.

Anthony Baldwin, currently CEO of AIG Europe Ltd., will become CEO of American International Group U.K. Ltd. and will continue to report to Mr. Townsend, the statement said.

AIG President and CEO Brian Duperreault, who rejoined AIG last May after more than 20 years running other insurance-sector companies, has instituted a turnaround plan for the struggling insurer that includes a revamp of its senior management.

Also Wednesday, Aspen Insurance Holdings Ltd. named Emil Issavi, president and chief underwriting officer of Aspen Re, to replace Mr. Lillelund in leading the reinsurance business, effective immediately.

Mr. Issavi will retain his current title, Aspen said in its statement. Mr. Lillelund is expected to remain with Aspen for a period of time to ensure a smooth transition, the statement added.