Thorn is the author and editor of numerous books, including Total Baseball: The Official Encyclopedia of Major League Baseball, Total Football: The Official Encyclopedia of Major League Football, Treasures of the Baseball Hall of Fame, The Hidden Game of Baseball, The Glory Days: New York Baseball 1947–1957, and The Armchair Book of Baseball. He founded Total Sports Publishing and served as its publisher from 1998–2002. Thorn served as the senior creative consultant for the Ken Burns documentary Baseball.
In 2004 Thorn discovered documentation that traced the origins of baseball in America to 1791 in Pittsfield, Massachusetts. In June 2006 SABR awarded Thorn its highest award, the Bob Davids Award. The award honors those whose contributions to SABR and baseball reflect the ingenuity, integrity, and self-sacrifice of the founder and past president of SABR, L. Robert "Bob" Davids.
On March 1, 2011, Thorn was named the Official Baseball Historian for Major League Baseball. "Thorn succeeds the late Chicago Tribune and Chicago Sun-Times baseball writer Jerome Holtzman, who served as Official Baseball Historian from 1999 until his death in 2008." Thorn's most recent baseball book, Baseball in the Garden of Eden: The Secret History of the Early Game, was published with Simon and Schuster on March 15, 2011.
Thorn is also the co-author with Pete Palmer and Bob Carroll of The Hidden Game of Football and with them co-editors of Total Football. His book New York 400, a graphical history of the city timed for its quadricentennial, created with the Museum of the City of New York and Running Press, was published in September 2009. Thorn is a columnist for Voices, the publication of the New York Folklore Society.
- MLB press release announcing Thorn's appointment
- Baseball: the writer's game. Retrieved March 30, 2011.
- George Robinson. "Batter Up, Historically Speaking". The Jewish Week. Retrieved March 30, 2011.
- "Q&A with John Thorn, baseball historian". StarTribune.com. Retrieved March 30, 2011.
- "Bob Davids Award – John Thorn". Society for American Baseball Research. SABR. Retrieved March 18, 2012.
- "John Thorn Named Official Baseball Historian".
- "Received: Baseball in the Garden of Eden". NotGraphs. Retrieved January 10, 2011.
- Our Game – Thorn's official MLB-hosted baseball blog
- Thornpricks – Thorn's family blog
- International Movie Data Base (imdb.com) – Thorn's appearances in TV and the movies