After graduating from Harvard and serving in the Army during World War I, Tunis began his writing career freelancing for American sports magazines while playing tennis in the Riviera.

For the next two decades he wrote short stories and articles about sports and education for magazines including Reader's Digest, The Saturday Evening Post and Esquire.

Tunis' eight-book baseball series about the Brooklyn Dodgers began with The Kid from Tomkinsville, a book often cited by sports writers and commentators as inspiring childhood reading.

After his death their mother taught at Brearley School for girls in Manhattan, later moving the family to Cambridge, Massachusetts, where she ran a boarding house.

[4] At age fourteen Tunis and his brother, too poor to pay the admission price, managed to watch a Davis Cup tennis match by climbing on top of a brewery wagon outside the courts.

[6] In 1921 the couple went to Europe where Tunis freelanced as a sports writer for American publications and played in some tennis tournaments on the Riviera, including a match against King Gustaf V of Sweden, who was 70 at the time.

"[5]: 269  Between 1920 and 1940 Tunis freelanced for a number of major magazines, including Reader's Digest, Harper's, Atlantic Monthly, Collier's, The Saturday Evening Post and Esquire, writing primarily on two topics: sports and education.

[9] Working six days a week and taking the seventh to play tennis,[5]: 271  Tunis published over 2,000 articles and short stories, becoming one ofAmerica's premier sportswriters.

An unflattering and thinly veiled fictionalization of tennis star Helen Wills Moody, it became the basis for the 1951 movie Hard, Fast and Beautiful.

"[5]: 261 As the Depression took its toll on magazine finances, Tunis began working on another novel, Iron Duke, the story of a small-town Iowa football star who struggles to fit in with his elite classmates at Harvard, and eventually finds strength through success as a runner.

In it, Tunis introduced rookie pitcher Roy Tucker and his teammates: "Bones" Hathaway, "Razzle" Nugent and "Fat Stuff" Foster.

"[25] Though his papers only list Tucker as "Number 36", they do say that, among others, "Gabby" Gus was based on Leo Durocher and Dave Leonard was inspired by Luke Sewell.

Million Miler, based on the life of TWA and U.S. Air Corps pilot Jack Zimmerman, was overshadowed by his other 1942 release, All American, called by Simon Certner in The English Journal "the most superb novel produced in its genre".

[29] In a chapter titled "John R. Tunis: The Best of the Best", Michelle Nolan's 2010 book Ball Tales praises All American as "a perceptive novel of character, of morals, and it's far ahead of its time".

"[30]: 188 With 1943's Keystone Kids, Tunis returned to his beloved Dodgers, again addressing anti-Semitism, this time as manager and shortstop Spike Russell struggles to get his brother, and the rest of the team, to accept star catcher Jocko Klein.

Keystone Kids received the Child Study Association of America Golden Scroll Award as the "most challenging children's book of the year".

"[5]: 262  In what Michelle Nolan in Ball Tales calls a "remarkable book",[30]: 184  Tunis uses his only non-Dodgers baseball novel to emphasize a favorite theme when the young protagonist admits to his father "it's better to lose, much as it hurts, than to play dirty".

Gail Murray in Boyhood in America called it "moving"[35] and Children Experience Literature said it was a "grimly realistic picture of warfare and its effect on both soldiers and civilians".

According to The National Cyclopaedia of American Biography, some critics consider A Measure of Independence "a powerful dramatic novel written under the guise of an autobiography".

[5]: 271–272  Horn Book agreed, calling it "his finest novel... With its irony and eloquence the story not only shows the futility of war but carries the central character to the heights of the protagonist in a Greek tragedy".

John R. Tunis, according to D. G. Myers "perhaps the greatest sports novelist of all time",[29] died on February 4, 1975, in Boston, Massachusetts, survived by his wife, Lucy Rogers.

[12]: 155 Leonard Marcus in Minders of Make-Believe: Idealists, Entrepreneurs, and the Shaping of American Children's Literature, says "Tunis's books were never only about sports", noting "the author's determination to offer his readers basic lessons about good citizenship and fair play, and a chance to reflect on such rarely discussed social issues as racial equality and anti-Semitism".

"[46]: 242  In turning from primarily writing non-fiction for adults to juvenile fiction Tunis did not abandon his emphasis on values over victory, but it did give him an audience that seemed more willing to listen.

'"[47] Many of Tunis' biggest heroes find themselves eventually brought low, like Roy Tucker in The Kid Comes Back, whose wartime service injury may have destroyed his career, or Iron Duke Jim Wellington at Harvard, ostracized and lonely, who perseveres by running track.

[25]: viii  And in what D. G. Myers in Best Baseball Books Ever called "one of the best pieces of (literary) criticism ever written",[52] The Kid from Tomkinsville is referenced by Nathan Zuckerman, the main character in Philip Roth's novel American Pastoral.

'"[54] Among Tunis' many childhood fans are sports writer and children's author Thomas J. Dygard,[55] Pulitzer Prize finalist Lee Martin,[56] journalist Charles Kuralt[57] and football legend Johnny Unitas.

[58] New York Post columnist and editor Pete Hamill picked The Kid From Tomkinsville as one of his five favorite sports novels, writing that "virtually every sportswriter I know remembers reading it as a boy.

"[59] In Partial Payment: Essays on Writers and Their Lives, literary critic Joseph Epstein devotes one chapter, "A Boy's Own Author", to Tunis.

[3]: 423 In The Continuum Encyclopedia of Children's Literature Nancy Horton called Tunis "the forefather of the genre of young adult sports fiction".

His stories gave the games context and addressed the pressures and problems of growing up in the spotlight, moving sports from the realm of pulp magazines to serious fiction.