Leonard Leopold Mackall, American bibliographer, editor and collector was born in Baltimore on January 29, 1879, the descendant of families prominent in Maryland and Georgia. He attended Lawrenceville preparatory school, where he graduated in 1896. In 1900 he received the degree of A.B. from Johns Hopkins University, and from 1900 - 1902 he attended the Harvard Law School. He studied at the University of Berlin from 1902 - 1904, was a fellow at Johns Hopkins University in 1906, and thereafter followed courses at the University of Jena. His interest in Goethe kept him for some years in Germany at Jena and Weimar, during which time he wrote in German a number of studies of Goethe's life and works. But these contributions were published in learned collections and periodicals and are known only to specialists. He edited "Goethe's Correspondence with Americans" for the Goethe - Schiller Archives at Weimar, and was co-editor of "Goethe's Collected Conversations," published in 5 volumes in Leipzig in 1910 - 1911. He contributed to the "Goethe Jahrbuch" and other German periodicals, as well as to the London "Athenaeum," "Modern Language Notes," "American Journal of Philology," the "American Historical Review" and to various other publications. On the outbreak of war in 1914 he returned to America. In 1916 he undertook the cataloguing of the Dr. Renne Georgia Library, an important collection of books relating to the history of Georgia.

Another of Mackall's special interests was the history of medicine. In the course of a number of visits to England he acquired a through knowledge of the library of Sir William Osler. When Sir William died, his Yale lectures on the subject of the evolution of modern medicine were still in proof, awaiting publication, and the editors called upon Mackall for aid in the verification of innumerable references. A short time later when work was begun on the catalogue of the Osler Library before its removal to McGill University, Mackall was asked to advise in the preparation of this important contribution to medical bibliography.

Mackall was chosen to edit the column "Notes for Bibliophiles" which began publication on September 21, 1924 in the New York Herald Tribune. He continued at this post for the remaining 13 years of his life. He was a member of the American Antiquarian Society of the Bibliographical Society and a member of the Bibliographical Society of England, as well as a number of other learned societies. He was president of the Georgia Historical Society, and in 1936 was president of the Bibliographical Society of America. Mackall's engrossing interest in books led to the collection of a rich variety of important works. An avid reader of booksellers' catalogues, he bought valuable books continuously, acquiring a library of considerable size and importance. He was also a generous donor, at various times giving to the Grolier Club of New York and to other libraries and book clubs in New York, Baltimore and Savannah large groups from his library which he considered were especially suitable. He bequeathed some 12,000 volumes to Johns Hopkins University.

Leonard Mackall had private means and was therefore able to pursue his studies and his collecting throughout his lifetime without the need of earning a livelihood. He died on May 19, 1937 in Fredericksburg, Virginia. He was unmarried.