His early interest in baseball derived from the Sunday afternoon games he attended with his father. After graduating from Morris High School in the Bronx, he worked in several jobs while developing a deep interest in baseball history, rules, and statistics. By the 1930s he was spending a lot of his free time in the New York Public Library researching old newspapers about the early accounts of matches. At that time, he developed a friendship with Ernest Lanigan, a baseball historian and Information Director of the International League.
Simmons worked for the International League from 1945 until 1966, first in New York then in Montreal. He then worked in the Baseball Commissioner's office until his retirement in 1982. He developed the playing schedules for the Majors and various minor leagues for over 20 years. Known as a historian and writer, he did much original research into 19th Century baseball. He developed and wrote the weekly feature "So You Think You Know Baseball," which ran in the Saturday Evening Post from 1949 to 1961. His book of the same name was a best-seller. For many years he wrote the entry for baseball in the Encyclopædia Britannica. In 1951, Simmons testified as an expert witness to the Celler Committee hearings on the history of the reserve clause. At the 1979 baseball winter meeting in Toronto, he was honored as "King of Baseball".
In 1990, his contributions to the game were recognized when he received the SABR Salute.
During the 1930s, Simmons developed a deep interest in baseball statistics and history. He was the first to compile 19th Century National League won-lost records for pitchers. The guides of that period had not published this information. He carefully checked each box score of each game listed in the newspapers of that era: Sporting Life and The Sporting News. The results were published over several issues of Baseball Magazine.
From 1940 to 1942, Simmons selected the top baseball performer of the day for the popular radio show "Fred Waring and his Pennsylvanians." While in the army he continued his research and while at Camp Pickett, Virginia, was able to work at the Library of Congress in Washington, where he compiled records from 1876, debuts of prominent players, batting records of pitchers, rare fielding gems and items for the Sporting News record book. He contributed original work to the top baseball writers of the day: J. G. Taylor Spink, Leonard Gettlson, Hy Turkin, S. C. Thompson, and Lee Allen.
In 1951, Simmons was called as an expert witness to testify before a congressional committee on the history of the reserve clause. The Celler House Judiciary committee probed monopoly influences in organized baseball.
In 1968 Simmons contributed a 26,710 word essay on the topic of Baseball which was printed in the Encyclopædia Britannica for many years.
International League (1946 - 1966) 
Simmons joined the International League during the 1946 season. On his first day on the job, Frank Shaughnessy asked him to go to Baltimore to present an award to Sherm Lollar for leading the league in hitting in 1945 with a .364 average. Simmons hopped on a train and in front of 45,000 fans made the presentation that afternoon. He indicated that he was extremely nervous speaking in front of a crowd that size.
Over the years he gained more responsibility in handling player trades, dealing with the press, scheduling the games, the hiring, firing and movement of the umpires, settling disputes among the clubs, handling the financial side of league operations, and staffing the office. During the late 1950s Shaughnessy became frequently ill, and Simmons was essentially running the league by himself.
In 1952, the league office was moved to Shaughnessy's home town of Montreal. Simmons quickly became a popular figure in Montreal baseball circles and gave many speeches to local community groups. He made many close friends among the sports writers and sport figures in both Montreal and Toronto. He moved his family north in 1954 to settle in the suburb of Cartierville, Quebec. The family retained this residence until 1995.
When the Montreal Royals folded in 1960, the league office moved back to New York City, but Simmons decided to keep his residence in Montreal. He would regularly spend 3 days a week in Montreal until his retirement from the Commissioners office in 1981.
Commissioner's Office 
His official duties in the Commissioner's office included supervision of club player contracts, co-ordination of National League and American League retirement plans, player service and pension records. He frequently was called upon for advice from the Commissioner and wrote speeches for the many functions attended by Commissioners William Eckert and Bowie Kuhn. He acted as a general consultant to club owners and general managers who needed advice.
As Jim Fanning, former General Manager of the Expos, wrote: "During Mr. Simmons time as a baseball executive every Major League owner and executive knew him on a first name basis. He not only was a keen advisor to the commissioners he worked for, but was a counsel and advisor to Major League Executives as well. Mr. Simmons was unheralded - his picture never made the cover of The Sporting News - but I had an office next to his when I started the Major League Scouting Bureau, and I witnessed this man's contribution day after day. His contributions were an equal to any who graced the cover of a sports magazine."
"So You Think You Know Baseball" 
When Simmons started at the International League, he held conferences for the umpires of the league to discuss the rules and review calls which had been made. He soon realized that some of the umpires were of the opinion that they "don't have to know the right answer unless the managers do". He started to compile some of these odd plays which the umpires would ask him about, and in 1949, he submitted "a pack of these nutcrackers" to the Saturday Evening Post under the title "So You Think You Know Baseball". The series became very popular; one of the solutions brought 7,000 letters of protest to the magazine. Nearly all were actually plays, "though a few were the results of bad dreams". The series ran until 1961, and later was published in Baseball Digest. In 1962, the series was published in book form and sold 500,000 copies in many editions.
One of Simmons' tasks at the International League was to develop the league schedule. In March 1953, when the Braves moved from Boston to Milwaukee, Warren Giles called on Simmons for some quick schedule changes. The next year found him developing both the American and National League schedules. He was the major league schedule maker from that date until 1982, when the required travel started to take its toll.
He also completed the schedules for numerous minor leagues, the Canadian Football League, soccer leagues, and international hockey tournaments.
Role in organizing the Montreal Expos 
Since Simmons had many friends in the baseball circles in Montreal, and knew everyone of importance in the game, it was only natural that he was called upon for assistance when Montreal was ready for a major league franchise. He directed Gerry Snyder of the mayor's office on how to go about getting the franchise and recommended the hiring of Jim Fanning and John McHale to run the club.
Other functions and accomplishments 
For many years, Simmons served on the Major League Rules Committee, where he suggested changes and wrote new rules.
In 1965, he appeared in the CBS Television show To Tell The Truth and managed to receive no votes when asked, "Will the real Mr. Simmons please stand up".
In 1979, Simmons was awarded the "King of Baseball" title at the annual Baseball winter meeting held in Toronto. This prestigious award is given annually to an individual who has made a major contribution to Major League baseball. It was generally felt that it was appropriately given to Simmons in Canada.
In 1990, he was awarded the SABR Salute which is given to a member whose research has contributed significantly to baseball knowledge.
He was inducted into the Canadian Baseball Hall of Fame in 2002, and in 2007, was elected to the International Jewish Sports Hall of Fame in Israel.
The Harry Simmons Collection at the Canadian Baseball Hall of Fame 
In the more than 50 years that he was involved in baseball, Harry Simmons collected thousands of items related to the development of the game from earliest times. These included his correspondence with people involved in every level of the game; memos, letters, and speeches from the commissioners office from the 1920s to the 1980s; notes and memos from his days as the major league schedule maker, letters from fans of his "So You Think You Know Baseball" series; articles from the International League; a significant collection of baseball memorabilia, and a major collection of baseball publications and books.
The collection was donated in 1998 to the CBHF&M by his son, David Simmons, who is a resident of Toronto. It has been called one of baseball's most eclectic, exciting and diverse collections. It will serve as a component of a future library and research facility which has been proposed for the CBHF&M.
- "So You Think You Know Baseball", Fawcett Publications, 1960, 1962, 1964
- Encyclopædia Britannica, article on Baseball, 1968
- Article from the New York Times, January 11, 1955, about scheduling
- New York Times, letter to the Editor, May 16, 1975
- New York Times, August 27, 1968
- New York Times, Nov. 30, 1955
- New York Times, Jan. 31, 1966
- New York Times, Aug. 2, 1951
- New York Times, Aug. 1, 1951
- New York Times, Dec. 3, 1956
- New York Times, Dec. 16, 1954
- New York Times, Dec. 2, 1964
- New York Times, Jan. 30, 1955
- New York Times, Sept. 24, 1980
- New York Times, Feb. 1, 1948
- New York Times, May 3, 1941
- New York Times, Aug. 2, 1951
- New York Times, Oct. 19, 1941
- New York Times, March 12, 1960
- New York Times, July 29, 1972
- New York Times, April 2, 1978
- New York Times, Dec. 10, 1980
- New York Times, June 2, 1980
- Canadian Baseball Hall of Fame entry
- SABR Biography Project
- International Jewish Sports Hall of Fame entry
- Article about Gerry Snyder
- Saber Salute article