The online mortgage provider Rocket Cos. is acquiring Mr. Cooper Group Inc. in an all-stock deal valued at $9.4 billion that will create a mortgage behemoth that handles one in every six mortgages in the US.

The combined company will service a book of $2.1 trillion of loans and nearly 10 million clients, according to a statement Monday. Mr. Cooper shareholders will receive 11 Rocket shares for each of Mr. Cooper’s stock they own, representing a 35% premium, it said in the statement.