The character of District Attorney Thomas Mara is clearly based on Thomas E. Dewey, a Manhattan District Attorney, who went on to become governor of New York state and twice the (unsuccessful) Republican candidate for President in 1944 and 1948. Jerome Cowan, the actor who played Mara and Dewey bear a strong physical resemblance and both wore mustaches, highly unusual for professional men of the time. Also, the Judge mentions that the District Attorney is a Republican, also a rarity back then for elected officials in New York City.
The Dutch girl spoke true Dutch, but with a heavy American accent.
The role of Kris was originally offered to Cecil Kellaway, who turned it down. The role went to Edmund Gwenn, Kellaway's cousin. Cecil Kellaway did play Santa in the Bewitched script A Vision of Sugar Plums, which featured child star Bill Mumy.
The scenes at Macy's were shot on location at the main New York store on 34th Street itself. Shooting was complicated by the fact that the crew's power needs exceeded the store's electricity capacity and required additional power sources arranged in the store's basement.
The scenes of the Macy's Thanksgiving Day Parade are of the actual parade held in 1946. As such, careful preparation was necessary for the shots as retakes were obviously out of the question. 20th Century-Fox had cameras positioned along the parade route at the starting line at 77th Street, on Central Park West, on the 3rd floor of an apartment building at 253 West 58th Street, in Herald Square and on 34th Street at 7th Avenue.
There are 21 mail bags carried into the courtroom at the end of Kris's hearing.
Unbeknownst to most parade watchers, Edmund Gwenn played Santa Claus in the actual Macy's Thanksgiving Day Parade held November 28, 1946. He fulfilled the duties of most parade Santas, including addressing the crowd from the marquee of Macy's after the parade was over. He was introduced to the crowd by actor Philip Tonge (he played Mr. Shellhammer in the movie) and he later unveiled the mechanical Christmas display windows to the accompaniment of Pyotr Ilyich Tchaikovsky's "Nutcracker Suite." This gesture symbolized the opening of the Christmas shopping season at the store.
When Dr. Pierce explains Kris' belief that he is Santa Claus, he offers for comparative purposes a Hollywood restaurant owner who believes himself to be a Russian prince despite evidence to the contrary, but rather conveniently fails to recall the man's name. This was a reference to Michael Romanoff, owner of Romanoff's in Hollywood, a popular hangout for movie stars at the time.