In 1914, Johnston became a reporter for the Fort Wayne Journal-Gazette. In 1916, he joined the staff of the Indianapolis Star as state editor. A year later he returned to the Fort Wayne Journal-Gazette as city editor. On December 1, 1917, he began a 45-year career at Purdue University. Four years after joining the Purdue staff, he established the Bureau of Information there and served as its director until retiring in 1963. In 1951, he went to Italy for a year to set up a farm education plan for the Marshall Plan. Johnston was an organizer of the Tippecanoe County Farm Bureau in 1919, the Greater Lafayette Chamber of Commerce in 1926 and the Hoosier State Press Association in 1930. He was also a member of the Lafayette Home Hospital board for 16 years and the West Lafayette Park Board for 14 years. Johnston was a 33rd degree Mason and belonged to many other fraternal and church organizations. He also served as Purdue's representative to the Indiana General Assembly and in 1956, took a leave of absence to seek the Indiana gubernatorial nomination, but was unsuccessful in his bid.