ST. PAUL — A developer says work on a $1 billion redevelopment of the old Ford assembly plant in St. Paul will begin early next year.

Ryan Companies will pay $61 million for the 122-acre site on a bluff above the Mississippi River in the Highland Park neighborhood. It plans a mix of office, retail and housing for the site where Ford shuttered its plant in 2011.

It could be three years before the first residents are able to move into the area and five years before the infrastructure and public amenities are completed, according to developer Mike Ryan.

Ford demolished the plant and cleaned up pollution left by nearly a century of industrial production.

Tony Barranco, senior vice president of development, said the price reflects Ford’s extensive cleanup work and the uniqueness of the parcel.

“In light of the investment they made in the cleanup of the site and the value of the real estate, I think we arrived at place that was fair to Ford, fair to the city and one that allows the project to be successful.”

The site will also have more than 55 acres of public and open spaces.