During the American Civil War, Jenkins served as a private with the 6th Wisconsin Volunteer Infantry Regiment. After the war, Jenkins became clerk of the Sauk County circuit court from 1867 to 1870. Jenkins moved to Chippewa Falls in 1870, where after some time reading law he became an attorney.
Jenkins served briefly in the Wisconsin State Assembly before becoming a judge for Chippewa County in 1872.
He was United States Attorney for Wyoming Territory from 1876 until 1880, after which he returned to Chippewa Falls and resumed his law practice. In 1894, Jenkins was elected as a Republican to Wisconsin"s 10th Congressional District.
He served as a Representative from that district for the 54th through the 57th Congress, after which he moved to Wisconsin"s newly created 11th Congressional District. He represented that district from the Fifty-eighth Congress to the Sixtieth Congress.
He served as Chairman of the House Judiciary Committee from 1903 to 1909.
In 1908, he was defeated in the Republican primary by Irvine Lenroot, who subsequently succeeded him as Congressman. In 1910, President William Howard Taft appointed Jenkins to serve a four-year term as Judge of the United States District Court for the District of Puerto Rico. Jenkins assumed that post in May 1910 but became ill and was unable to perform much judicial work.
He died the following year in Chippewa Falls on June 8, 1911 and was succeeded by Paul Charlton.