He was the son of William Henry Llewellyn, a businessman of Welsh ancestry, and Janet George, a passionate suffragette and prohibitionist of Congregationalist conviction.

At the age of sixteen he was sent to study in Germany, at the Realgymnasium of Schwerin, where he spent three years and passed his Abitur (school-leaving examination) in the spring of 1911; he learned to speak an excellent German and was able later in life to publish in that language.

[2][4] At Yale he got acquainted with two prominent law professors and key figures of the incipient legal realism movement, Arthur L. Corbin and Wesley N. Hohfeld, whose influence on him was profound.

After spending ten weeks in a German hospital at Nürtingen and having his petition to enlist without swearing allegiance to Germany turned down, Llewellyn returned to the United States and to his studies at Yale in March 1915.

As one of the founders of the U.S. legal realism movement, he believed that the law is little more than putty in the hands of a judge who is able to shape the outcome of a case based on personal biases.