American Capital shareholders will receive $6.41 per share in cash and 0.483 Ares Capital shares for each share held.
The combined companies would have more than $13 billion in investments at fair value as of March 31, Ares said in a statement.
The deal does not include the sale of American Capital Mortgage Management, which would be sold separately to American Capital Agency for $562 million.
Wells Fargo Securities and Bank of America Merrill Lynch were financial advisers to Ares Capital. Goldman Sachs and Credit Suisse Securities LLC advised American Capital.