They soon established an educational department and quickly expanded the company’s educational offerings. Beginning with the publication of the Stanford-Binet Intelligence Scale during World War I, Houghton Mifflin became increasingly involved in publishing standardized tests. The Riverside Publishing Company was officially established as a wholly owned subsidiary of Houghton Mifflin in 1979.

In early 2000 Houghton Mifflin began a very complex restructuring and acquisition process and emerged as Houghton Mifflin Harcourt.