Ernest graduated magna cum laude with a major in philosophy in 1885. At Harvard he edited the college humor magazine, the Harvard Lampoon. The eminent American philosopher William James was a teacher and friend. Other classmates included William Randolph Hearst and George Santayana.
After college, and typical of the sons of the well-to-do, Ernest went abroad and settled for a time in Paris. Despite his father's desire to have him work in the family business, Ernest took a job writing humor pieces for his college friend Hearst, who was now running the San Francisco Examiner newspaper. Returning to Worcester in 1888, Thayer wrote "Casey" in May and Hearst published it in the June 3, 1888 edition of his newspaper. Thayer wrote his columns for the newspaper using the pseudonym "Phin" and it would be several years before the true authorship of "Casey" would be determined.
Thayer eventually went to work for his father but ultimately quit altogether when he moved to Santa Barabara in in 1912. It was in California, at age 50 that he married Rosalind Buel Hammett, a widow from St. Louis. They had no children.
Described as a slightly built, soft-spoken man who wore a hearing aid after middle age, Thayer died in Santa Barabara, in 1940.
In his brief review of Thayer's life, Martin Gardner writes:
One might argue that Thayer, with his extraordinary beginning at Harvard, his friendship with James and Santayana, his lifelong immersion in philosophy and the great books, was himself something of a Casey. Just before Thayer died he attempted to put some thoughts down on paper. However, he was too old or too sick to carry out the task and he lamented, "Now I have something to say and I am too weak to say it."Nevertheless, Thayer will forever be remembered for one remarkable at bat, a tragic-comic hit about a mighty hero who struck out.