The New York Yankees and Los Angeles Dodgers were two of John's former clubs offering condolences after he died at his home in Bradenton, Florida, where he had received hospice care after a recurrence of bladder cancer.
"We are saddened to hear the news of Tommy John's passing today," Dodgers president Stan Kasten said in a statement.
"Tommy was an exceptional pitcher throughout his career in Major League Baseball, and his courageous role in becoming the first to have surgery that would go on to bear his name can't be overstated.
"His impact both on and off the field has been felt by ballplayers of all ages and will be for generations to come."
Current Dodgers superstar Shohei Ohtani is just one of the MLB pitchers who has undergone Tommy John elbow ligament reconstruction surgery.
The procedure has also saved the careers of Tarik Skubal, Justin Verlander, Chris Sale, and countless other pitchers around the league both past and present.
John was pitching for the Dodgers on July 17, 1974, when he threw a fastball and felt what he called a "crash" in his left elbow.
The injury was a career-threatening tear to his ulnar collateral ligament.
Treatment with ice and rest failed, and Dodgers team doctor Frank Jobe performed a then-revolutionary procedure on September 25, 1974, in which he harvested a tendon from John's right forearm and attached it to bones in his left elbow.
John required a second procedure to repair nerve damage, but after missing all of 1975, he beat 1-in-100 odds to return to the mound, going 10-10 with a 3.09 earned run average in 31 starts for the Dodgers in 1976.
Dubbed the "Bionic Man," John would finish second in National League Cy Young Award voting in 1977, building a 20-7 record to help the Dodgers claim the NL pennant.
"I was able to get hitters out when I didn't have good stuff; my arm was like I never had surgery," John later said.
John, who started his MLB career with the Cleveland Indians in 1963 and played for the Chicago White Sox from 1965-71 before moving to the Dodgers, had two stints with the Yankees, from 1979-82 and from 1986-89.
"The New York Yankees mourn the passing of Tommy John, who was a bedrock of our rotation during his time in pinstripes and a longtime favorite of Yankees fans," the Yankees said in a statement.
"While his incredible longevity and post-surgery achievements changed the fortunes and trajectory of countless athletes around the world, it also overshadowed his excellence as a pitcher.
"We pause to remember the totality of his legacy — his competitiveness and skill, his penchant for conversation, his kind and gracious nature, and his stature as an athletic and medical pioneer."
John, born in Terre Haute, Indiana, on May 22, 1943, was signed by the Indians when he was 18 and made his MLB debut two years later.
He finished his career with a 288-231 win-loss record and an earned-run-average of 3.34, with 2,245 strikeouts.
He played in three World Series - in 1977 and '78 with the Dodgers and 1981 with the Yankees - but was on the losing side each time.
After his playing career ended, John worked as a broadcaster for the Minnesota Twins and the minor-league Charlotte Knights. He also spent time coaching.
On August 8th, the Yankees had posted a "goodbye letter" written by John on social media, with the team saying he was battling "health issues".
In the letter, John thanked friends and fans and Dr. Jobe, "who saved my arm & made it possible for me to continue pitching."