Cliff Hillegass was born in 1917 in the Great Plains of Nebraska in the town of Rising City. He graduated from Rising City High School, entered Midland College in Fremont, Nebraska, and, after graduation, began graduate work as an assistant in geology at the University of Nebraska in Lincoln.

After a stint in the Army Air Corps in the 1940s Hillegass began work at Long's Book Store, which soon was renamed Nebraska Book Company, a firm that is among the nation's largest handlers of used books. He rose rapidly through the ranks and became a buyer of used textbooks throughout a large portion of the United States. Because of the company's widespread wholesale network, his enthusiasm, and his grasp of the value of books, Hillegass became friends with bookstore managers from coast to coast. These close business friendships became the lynchpins that would later help him launch Cliffs Notes.

In the late 1950s, while Hillegass was visiting Jack Cole, a Canadian book dealer, Cole suggested that Hillegass publish a line of literary study guides similar to Cole's Notes. Cliff liked the idea, returned to Nebraska, borrowed money, and arranged with Boomers Printers to publish 4,000 copies each of sixteen of Cole's top-selling Shakespeare titles. Many book dealers were dubious about the venture, but, because of their longtime friendship with Hillegass, they agreed to stock Cliffs Notes, which were boxed and shipped from Hillegass's basement in Lincoln, Nebraska.

CliffsNotes were an immediate success. So much so that the business moved from the basement to a large warehouse in downtown Lincoln. The series continued to prosper, doubling in sales every year, and numerous employees were added to the firm, including professional editors, a consulting editor, and many university professors as writers. Along the way, competing firms began selling literary study guides (in the early 1970s, for example, there were thirteen competitors), but Cliffs Notes outsold them all and survived to become the generic term for all study guides.

Today, Cliffs Notes are sold all over the world, and many of the titles have been translated into other languages. In the United States they are often used as a teaching tool in high schools and are recommended or required reading for many university graduate courses. Cliffs Notes remained a Lincoln, Nebraska, firm until it was acquired by IDG Books Worldwide, Inc., in 1998 and operations were moved to Indianapolis, Indiana. Cliff Hillegass died in Lincoln, Nebraska, on May 5, 2001.