This series consists of the family and personal correspondence of the families of John White Hallowell and Marian Ladd Hallowell. The earliest letters are those written from 1859 to 1861 to Robert and Hannah Haydock, Marian Hallowell's maternal grandparents, from Hannah's cousin John Corlis White of New York. White writes first as a student at Harvard University, then as a member of the 7th New York Volunteer Infantry Regiment on his way to defend Washington, D.C. in April 1861, mentioning Robert Gould Shaw as a member of his unit. In several letters written in May of 1861, he describes daily activities at his encampment at the U.S. Capitol in Washington and later at Camp Cameron in Georgetown Heights. Another early letter is that of Col. Edward N. Hallowell of the 54th Massachusetts Volunteers, informing John Ritchie of the death of his son Edward in April 1865.

The bulk of family correspondence is that of John and Marian Hallowell. Other frequent correspondents include John's parents, Norwood Penrose Hallowell and Sarah Haydock Hallowell, and his siblings Anna Norwood Hallowell Davis (1871-1943), Esther Fisher Hallowell Morse (1881-), Susan Morris Hallowell Brooks (1882-1985), and Robert Haydock Hallowell (1873-1958), who largely discuss daily life, social gatherings, and family activities. Many pre-1905 letters are courtship letters between John and Marian. Correspondence from Marian's friends and family before 1905 mainly discuss courtships and engagements within her circle of acquaintances. After 1905, much of the correspondence consists of congratulations for John and Marian's marriage and the births of their children, as well as condolences for the death of Marian's sister Elanor Ladd Clark in 1912 and of Norwood P. Hallowell in 1914. Later letters from Marian to John while he traveled for business recount her daily life in Milton with the children and visits from family and friends. John's letters to Marian recount his business travels, primarily to the American South and Midwest. There are few letters from John after 1914.

Friends and family members describe experiences from their trips abroad, particularly in Rome and Paris. Later letters written by John and Marian's sons William and Roger discuss travel, college, and life in Milton. Of interest is an 1898 letter discussing the Spanish American War in Cuba; a letter to Marian from Maya Langsley describing her 1900 trip to Yokohama, Japan; a March 1908 letter from Marian to John recounting a trip to the White House in which she met President William Howard Taft; and a copy of a March 1914 letter from A. Lawrence Lowell to Norwood P. Hallowell about segregation in Harvard dormitories. Letters from American diplomat William Phillips to John Hallowell, largely written in London from 1908 to 1912, primarily contain news of family and friends.

Papers include correspondence reflecting Hallowell's positions as a director of the Boston Chamber of Commerce and trustee of the Middlesex School of Concord; his position in the securities department of Stone & Webster, Inc.; and his memberships in the Milton Club, the Union Club of Boston, and the Harvard Club of Boston. Financial papers include purchase agreements and receipts for an automobile and portable garage, insurance receipts, and papers related to Hallowell's 1914 purchase and maintenance of a summer home on Wing's Neck in Bourne, Mass. Of note is an October 1914 letter from business associate William Maxwell describing London in the grips of the First World War.

Much of John’s Harvard-affiliated correspondence consists of letters from Edgar H. Wells, general secretary of the Harvard Alumni Association, as well as letters from officials of the Harvard University track team, the Harvard Athletics Association, and other university organizations. Other Harvard papers include Hallowell's 1897-1898 report card, notes for his 1926 speech at the 25th reunion of the Harvard Class of 1901, and a 1927 Harvard Bulletin containing his obituary.

Printed material includes newspaper clippings containing the 1914 obituary of Norwood P. Hallowell, postcards containing photographs of the volunteer training program at the Plattsburgh (N.Y.) camps before the U.S. entry into World War I, and miscellaneous undated ephemera.