Seth H. Yocum was born in Catawissa, Columbia County, Pennsylvania on August 2, 1834. He was educated in rural schools and then went to Philadelphia to learn the printing and editing trade. Yocum entered the class of 1860 at Dickinson College in Carlisle, Pennsylvania. He was a member of the Phi Kappa Pse fraternity and graduated with his class in the early summer of 1860. He then returned to Philadelphia, where he was employed as an editor.
In July 1861, in Philadelphia, Yocum enlisted in Company C, Fifth Pennsylvania Cavalry as a sergeant. He transferred to Company A as second lieutenant in February 1862 and to Company G as first lieutenant in November 1862. Yocum mustered out in September 1864 at the end of his three-year enrollment and took up law studies. He was admitted to the Schuylkill County bar in Pottsville in 1865 and opened a practice.
Yocum relocated to Bellefonte, Pennsylvania in 1874 to join the firm of Bush, Yocum, and Hastings to replace his brother George, who had just been killed in a hunting accident. He served as the district attorney of Centre County from 1875 until 1878, when he won election as a member of the Forty-sixth Congress for the Twentieth District. Yocum was one of twenty-one independent members elected that year. He stood as a member of the Grange-influenced Greenback-Labor Party and defeated fellow Dickinsonian and former governor, Andrew Gregg Curtin, to win the seat. Yocum declined to serve more than one term and moved to Johnson City, Tennessee, where he involved himself in the large tanning concern of his father-in-law. He also served as the mayor of Johnson City in 1885. Soon after, Yocum moved his family to Pasadena, California and invested in the beginnings of the orange growing in the area. True to form, he served as mayor of Altadena while there.
In January 1867, Yocum married Lucinda Horton of New York. The couple had five children who survived infancy. On April 19, 1895, Seth Hartman Yocum died at his home in Pasadena and was later buried in the Mountain View Cemetery in the city. He was sixty years old.