Ernest Lawrence Thayer (August 14, 1863 - August 21, 1940) was an American writer and poet who wrote "Casey at the Bat".

Thayer was born in Lawrence, Massachusetts and raised in Worcester. He graduated magna cum laude from Harvard in philosophy in 1885 after editing the Harvard Lampoon. Its business manager, William Randolph Hearst, hired Thayer as humour columnist for the San Francisco Examiner 1886-88.

Thayer’s last piece, dated June 3, 1888, was a ballad entitled "Casey" ("Casey at the Bat").

It took two decades for the poem to make Thayer famous, as he was hardly the boastful type and had signed the June 3 poem with the nickname "Phin". Two mysteries remain about the poem: who, if anyone, was the model for the title character and whether Thayer had a real-life "Mudville" in mind when he included Mudville as the poem's mythical town. On March 31, 2004, Katie Zezima of The New York Times penned an article called "In 'Casey' Rhubarb, 2 Cities Cry 'Foul!'" on the competing claims of two towns to such renown: Stockton, California, and Holliston, Massachusetts.

As far as whether there was any model for the title character, Thayer later dispelled the notion that any single living baseball player was an influence. However, late 1880s Boston star Mike "King" Kelly is odds-on the most likely model for Casey's baseball situations. Besides being a native of a town close to Boston, Thayer, as a San Francisco Examiner baseball reporter in the offseason of 1887-88, covered exhibition games featuring Kelly. In November 1887, some of his reportage about a Kelly at-bat has the same ring as Casey's famous at-bat in the poem. A 2004 book by Howard W. Rosenberg, Cap Anson 2: The Theatrical and Kingly Mike Kelly: U.S. Team Sport's First Media Sensation and Baseball's Original Casey at the Bat, reprints a 1905 Thayer letter to a Baltimore scribe who was inquiring about the poem's roots. In the letter, Thayer singled out Kelly, who had died in 1894, as having shown "impudence" in claiming to have written it. Rosenberg argues that if Thayer still felt offended, Thayer may have steered later comments away from connecting Kelly to it. Kelly had also performed in vaudeville, and recited the poem dozens of times, possibly butchering it to Thayer's dismay. Incidentally, the first public performance of the poem was on August 14, 1888, by actor De Wolf Hopper, on Thayer's 25th birthday.

Thayer's recitation of it at a Harvard class reunion in 1895 may seem trivial except that it helps solve the mystery, which lingered into the 20th century, of who had written it. In the mid-1890s, Thayer contributed several other comic poems for Hearst's New York Journal and then turned to make his livelihood by overseeing his family's mills in Worcester.

He moved to Santa Barbara in 1912, where he married Rosalind Buel Hammett and retired. Thayer died in 1940, at age 77.