Hanson's father was a government employee and a landlord, but his family was big and profitable prospects in Norway were hard to come by.

Hanson's mother's half-brother Han Roberg, who had settled in Decorah, Iowa, offered education to one of the boys of her family, and nine-year-old Jens was selected.

Lack of finances forced him to drop out after two years, and in 1890, he joined the workforce of the newly organized Newberry Library in Chicago.

This task, begun during the short authority of John Russell Young as Librarian of Congress and finalized under his successor, Herbert Putnam, involved the constructing of a new classification system for the library's volumes and the invention of a new roster.

In applying this principle to a collection of similar unaccustomed size and complexity, however, Hanson created numerous expansions which were incorporated into the cataloging regulations.

In 1910, Hanson relocated to the University of Chicago as associate director of its libraries, which he reorganized to attain bibliographical regulation in a greatly decentralized network.

At the same time, he traveled to Italy, where he led a team of American experts who aided in the reorganization of the Vatican Library in Rome.

Hanson was a member of the Bibliographical Society of America, the American Library Association (ALA), Kappa Sigma, and the Quadrangle, Norske and University organizations.