Karsner is best remembered as a key member of the editorial staff of the New York Call and as an early biographer of Socialist Party of America leader Eugene V. Debs.
[2] While in Chicago Karsner made the acquaintance of a number of socialist intellectuals, including Upton Sinclair, Jack London, and Carl Sandburg.
[2] One of the major stories covered by Karsner during his time at The Call was the 1918 mass trial of 166 members of the Industrial Workers of the World held in Chicago before Judge Kenesaw Mountain Landis.
[2] In April 1923 Karsner resigned from the financially struggling Call in protest over the paper's decision to publish a critique of Soviet Russia written by Francis McCullaugh, a member of the British secret service.
[3] Disaffected from the increasingly conservative Socialist Party, Karsner turned to writing non-fiction, authoring biographies of President Andrew Jackson and radical abolitionist John Brown.