The deal for the group, which includes equity release adviser Key Retirement, lender More2life, and referral service Key Partnerships, is worth £208m.
The business also announced that founder and long-time CEO Colin Taylor will retire after 20 years in the post.
He will be succeeded as CEO by Simon Thompson, who recently served as deputy CEO.
Partners Group has targeted significant growth for the business following its acquisition, with it focusing on expanding distribution capacity, broadening the product portfolio and driving product innovation.
Key Retirement Group CEO Simon Thompson said: “Colin’s drive and ambition have been crucial in making Key Retirement Group the success that it is and we wish him all the best on his well-deserved retirement.
“We are very pleased to finalise the deal with Partners Group and look forward to starting work with them as our new owner, building on the significant growth achieved over the last few years with Phoenix who were outstanding partners.”