The deal to acquire the equity release specialist was first announced in January of this year.
Chris Evans, group chief executive, MGM Advantage group, said: "This acquisition provides us with a strong platform to take advantage of the market changes brought about by the Budget.
"We see huge potential in the market as customers seek innovative solutions to long-term retirement planning. "Stonehaven is a key part of our strategy to make the most of the changes, and by sharing ideas and expertise across the group, we are accelerating our product development in the post-Budget world."

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The companies have said that Stonehaven, which becomes a wholly owned subsidiary, will retain its brand and continue to focus on the lifetime mortgage market, selling its products through financial intermediaries.
Georgina Smith, managing director, Stonehaven, said: "The landscape might be changing since the Budget but the fundamental need for people to secure an income in retirement still remains.
"Property wealth will play an increasingly important role, with equity release providing the back-up plan when people realise they don't have the income to support their living needs.
"The market is set to grow quickly over the next few years, as people enter retirement with interest only mortgages.
"People will be trying to balance the financial needs of the family, with many also seeing a shortfall on their chosen repayment vehicle.
"Lifetime mortgages can offer a sensible and flexible solution."