The Mary Boyle O’Reilly papers document her work as a journalist and political activist, particularly during World War I. They include O’Reilly’s correspondence from Europe during the war, the bulk of which relates to her journalistic work. A large portion is addressed to "the Chief," Sam T. Hughes, editor in chief at Newspaper Enterprise Associates of America, a syndicate that supplied content to newspapers across the United States. These letters frequently include portions of her articles. The collection also includes O'Reilly's typescripts. The bulk of these are short writings on her experiences during the war. There are indications that some may have been written at the time but others retrospectively; while most works are dated between 1914 and 1917, some contain the annotations "memories" or "verify date." Her writings also include a 1909 article on the 1834 Ursuline convent burning in Charlestown, Massachusetts, and a few longer pieces on politics in Europe, dating from the 1930s. A small series of individually typed or handwritten poems, many with inscriptions from O'Reilly's friends, and childhood photographs of her father, sisters, and birth home round out the collection. Materials include typescripts, photographs (including cabinet cards and photomechanical prints), correspondence, newspaper clippings, and a postcard.