Many facts were changed or ignored to add to the feel of the movie. For example, the real George M. Cohan was married twice, and although his second wife's middle name was Mary, she went by her first name, Agnes. In fact, the movie deviated so far from the truth that, following the premiere, the real George M. Cohan commented, "It was a good movie. Who was it about?"
Despite failing health, the real George M. Cohan acted briefly as a consultant on the film. He lived long enough to see the finished result and approved wholeheartedly of James Cagney's depiction of himself.
James Cagney hated working with character actor S.Z. Sakall, who he felt upstaged him at every turn. He complained to director Michael Curtiz about him but Curtiz took no action, as he had great respect for Sakall, who was once Hungary's leading actor. Cagney refused to work with Sakall again.
Joan Leslie portrays Mary Cohan, aging from 18 to 57 throughout proceedings. Leslie turned 17 during the production of the film. The fact that she was still attending school during production caused numerous delays.
In the film George M. Cohan writes a drama called "Popularity", which was a failure. This is fact. Following WW I he rewrote parts of it, added music, and put it on under the title of "Little Nelly Kelly", which became a huge hit.
Carl Jules Weyl's theater stage set took up a whole sound stage and was specifically constructed so that it could replicate the proscenium design of any given theater, from the traditional 19th-century stylings of the Liberty (now Madame Tusseud's Wax Museum, where "Little Johnny Jones" opened in 1904) and Herald Square (demolished in 1915, where "George Washington Junior" opened in 1906) Theaters, to the Art Deco design of the Alvin (now the Neil Simon, where "I'd Rather Be Right" opened in 1937) Theater.
Among the opening credits is one that states "Music and Lyrics by George M. Cohan." However, the come-back musical "I'd Rather Be Right" has a score by Richard Rodgers and Lorenz Hart, including the romp, "Off the Record", one song decidedly NOT by Mr. Cohan. The stylized Central Park set depicted on the stage is, in fact, a recreation of the original Broadway set as seen in historical photographs.
Although 'Josie Cohan Niblo' did predecease Jerry Cohan, dying of a diagnosed heart condition in 1916 at the age of 40 (her then 13-year-old son, future screenwriter Fred Niblo Jr., discovered her body in an upstairs hallway), his wife, Helen Costigan "Nellie" Cohan, actually survived him by 11 years, dying in 1928. In the film it is stated that both Jerry Cohan's daughter and wife predeceased him.
According to James Cagney's autobiography his brother William Cagney (who was also his manager) actively pursued the role of ultra-patriotic George M. Cohan for James as a way of removing the taint of James' political activities in the 1930s, when he was a strong, somewhat radical supporter of President Franklin D. Roosevelt. When Cohan himself learned about Cagney's background as a song-and-dance man in vaudeville, he approved him for the project.
This was the very first black and white movie to be colorized using a controversial computer-applied process. Despite widespread opposition to the practice by many film aficionados, stars and directors, the movie won over a sizeable section of the public on its re-release.
Frances Langford is listed in the credits simply as "Singer". In the film, Cagney calls her "Nora", so this character is probably the real-life Nora Bayes (1880-1928). Bayes was a popular performer who recorded many Cohan songs and entertained the troops with Cohan during World War I. Bayes wrote the song "Shine on Harvest Moon" and was the subject of the Warner Brothers biopic Shine on Harvest Moon (1944). In "Yankee Doodle Dandy", Langford also sings the medley "In a Kingdom of Our Own" / "Love Nest" / "Nellie Kelly, I Love You" / "The Man Who Owns Broadway" / "Molly Malone"/ "Billie" that backs up one of Don Siegel's great montage sequences. Langford sang "Over There" to WW II American troops and toured with Bob Hope to entertain American troops in WW II, Korea and Viet Nam.
George M. Cohan died of cancer shortly before the film was released. However, he got a private screening. His comment on the film (and James Cagney playing him) was "I wouldn't want to follow that act."
The car the college kids are driving is a 1933 Chevrolet Phaeton "Jalopy". Graffiti from back to front reads: "Exit Here" (arrow pointing to door handle), "Open Here", "Will Stop Quick if a Wheel Brakes", "For Sale", "Frankie & Johnnie", "But Good", "In Case of Fire Scream".
George Tobias, who played Dietz of the theatrical production team of Dietz and Goff later played Abner Kravitz, husband of nosy neighbor Gladys Kravitz in the 1960's television sitcom series Bewitched.
George M. Cohan was awarded the Congessional Gold Medal by Act of Congress (not the Congressional Medal of Honor as stated in the film). He was as said in the film the first member of the entertainment profession to receive this award. Subsequent entertainers to receive the medal are: Irving Berlin, Walt Disney, Bob Hope, Danny Thomas, John Wayne, Marion Anderson, Frank Sinatra, Harry Chapin, Fred Waring, and Aaron Copeland.
The Yankee Doodle Dandy number in the movie was from Cohan's first Broadway production, "Little Johnny Jones", reputed to be the first American musical. It was based on events in the life of the American jockey Tod Sloan, who in 1903 went to London to ride in the English Derby and was accused of throwing the race.