At the age of two, in 1839, Seymour's father died in a yellow fever epidemic in New Orleans.

[2] At the age of 17 he began an apprenticeship at Novelty Iron Works of New York and was subsequently employed by the Erie Railroad.

When he was 21, in 1858, he was appointed master mechanic of a railroad from Matanzas to Puerto Príncipe in Cuba, and later worked for two years as chief engineer of a large Cuban sugar plantation.

After the death of his first wife, he married her sister Anna J. Crowell, and the couple also had one son, David C.

[2] Seymour argued for the creation of "Greater Newark" by the annexation of a number of nearby towns: East Orange, Vailsburg, Harrison, Kearney, and Belleville.