The Eternal City, which marked stage star Pauline Frederick's screen debut, was shot on location in England and Italy and included many scenes of Rome's most famous edifices. Miss Frederick left New York for Rome on the Imperator on 6 June 1915, according to the 20 June 1915 Motion Picture World. As noted in the 1 August 1914 Motion Picture News, "For the first time in the history of motion pictures they succeeded in filming the Vatican, the Coliseum, and in fact every historical and beautiful spot in the Imperial City. It is the first time that the authorities of Rome have given permission for the ancient city to be put upon the screen." The directors also used thousands of soldiers and hundreds of railway and government officials. However, for interiors, the production moved back to New York City and filmed at the Famous Players' studios on West Twenty-sixth Street. Reviewer George Blaisdell, writing in the 9 January 1915 Moving Picture World, marveled at the scenes of "the Pope's jubilee—the great Plaza, the winding religious processions....Most remarkable of all, however, are the views of the great public meeting in the Coliseum....The upper tiers of the immense roofless walls are lined with soldiers; down below, many hundreds and seemingly several thousand men are crowded about David Rossi."
       The costumes of the Noble Guard of the Roman Pontiff and the Swiss and Municipal Guards were reproduced for the film. The August 1915 Picture-Play Magazine reported that the directors used two scenarios when they filmed near the Vatican. "One was the plot of a tale about a lost girl being redeemed by the church. The other was the real scenario. The first was shown to all inquirers, for the Swiss Guards would have objected to the filming of The Eternal City, but they allowed scenes to be taken around the Vatican and St. Peter's [Cathedral] for what they thought was a 'lost girl' tale." Edwin Porter told the 5 June 1915 Pictures and the Picturegover, a British magazine, that a scene in which the "David Rossi" character runs through the streets of Rome and finally falls upon the steps before the bronze gates of the Vatican, was taken early on a Sunday morning "when there were no sightseers around. The Swiss Guards were watching us from the inside, but I knew they wouldn't come out to interfere. Just then a priest came out of the Vatican, saw the prostate man, and, not realising [sic] what the idea was, bent over him and raised his head. That was just what I wanted, but through some mishap didn't get it right, and [co-director Hugh] Ford saved the day for me. He rushed right up to the priest and said excitedly, 'Do that again, please.' The priest obeyed automatically, without knowing just why he did it, and I got a splendid picture."
       An elaborate premiere was held at New York's Lyceum Theatre on 27 December 1914, but the film was not released until the following year. It had its first public showing in New York on 12 April 1915. In order to handle such a long and expensive film, Famous Players organized The Select Film Booking Agency to distribute it, along with other films that were too long for their regular program. The prices of admission began at twenty-five cents.
       An edited, five-reel version of The Eternal City was released on 15 December 1918, as part of Paramount's Success Series.
       The National Film Preservation Board (NFPB) included this film on its list of Lost U.S. Silent Feature Films as of January 2021.