He served as Kokomo's city attorney from 1871 to 1884. Kern was elected to the Indiana Senate in 1893, serving for four years, serving at the same time as assistant U.S. Attorney for Indiana. From 1897 to 1901 he was city solicitor of Indianapolis. He was the unsuccessful Democratic candidate for Governor of Indiana in 1900 and 1904. After these defeats, he returned to his law practice, traveled to Europe, and spent six months at a sanatorium in Asheville, North Carolina, for reasons of health. In the 1908 election, he was the Democratic candidate for Vice President, running mate to third-time Presidential candidate William Jennings Bryan as a Midwestern compromise. Bryan was defeated by Taft. Kern then sought election to the United States Senate from Indiana, but was outmaneuvered by fellow Democrat Benjamin F. Shively.
United States Senate
Indiana's other Senate seat came up for election in 1910, and this time the legislature elected Kern. He entered the Senate in 1911, one of ten new Democrats—most of them progressives. Joining Shively, Kern became a progressive Democrat and an opponent of monopolistic corporate power. He quickly became involved in an effort to shake up his party's conservative leadership. In 1912, he helped write the Democratic platform, which had progressive planks in favor of banking and tariff reform, and direct popular election of Senators. In the election of 1912, Woodrow Wilson was elected president, Democrats gained a majority in the House, and eleven more progressive Democrats entered the Senate. Kern's national stature as a progressive, his skill at conciliation, and his personal popularity resulted in his unanimous election as Chairman of the Democratic Caucus and de facto majority leader. He worked closely with President Wilson and often met with him privately. He kept the peace and promoted unity that helped propel Wilson's initiatives through the Senate. These included tariff reform, the nation's first income tax, the Federal Reserve Act, antitrust laws, and the Federal Trade Commission. In 1913, Kern was contacted by labor activistMary Harris Jones, who had been imprisoned by a military court in West Virginia during the Paint Creek-Cabin Creek strike of 1912. In response, Kern introduced the Kern Resolution, adopted by the Senate on May 27. The resolution led to the Senate Committee on Education and Labor investigation into conditions in West Virginia coal mines. Congress almost immediately authorized two similar investigations: into conditions in copper mining in Michigan and coal mining in Colorado. Kern had advocated direct popular election of Senators, and helped enact the 17th Amendment to establish it in 1913. However, when Kern sought re-election in 1916 under the new system, he was defeated by Republican Harry S. New, narrowly losing the popular vote.
Retirement and death
At Bryan's urging, Wilson considered Kern for appointment to various offices, but Kern was in poor health and unable to serve. He died on August 17, 1917, in Asheville, five months after leaving the Senate. He was originally interred at his summer home near Hollins, Virginia, and re-interred in Crown Hill Cemetery in Indianapolis twelve years later. He was survived by his wife Araminta C. Kern, who died at age 85 in 1951, and his son John W. Kern Jr., a future judge and mayor of Indianapolis.