Eager was born in and grew up in Toledo, Ohio and attended Harvard University, class of 1935. After graduation, he moved to New York City, where he lived for 14 years before moving to Connecticut. He married Jane Eberly in 1938 and they had a son, Fritz.

Eager was a childhood fan of L. Frank Baum's Oz series, and started writing children's books when he could not find stories he wanted to read to his own young son. In his books, Eager often acknowledges his debt to E. Nesbit, whom he thought of as the best children's author of all time.

A well-known lyricist and playwright, Eager died on October 23, 1964 in Stamford, Connecticut, at the age of fifty-three.