Clark was born July 17, 1823 in Wakeman, Huron County, Ohio, where he spent his childhood on his family's farm. He was educated in the public schools and at the Academy of Oberlin College. In 1846 he moved from Ohio to Port Washington, Wisconsin where he worked as a surveyor, in a drug store owned by an older brother, and as deputy sheriff. In 1849, he undertook an overland journey to California to join the Gold Rush there. In California, he engaged in mining, packing, and trading, principally in the vicinity of Shasta and Yreka. He returned east via sea and the isthmus of Panama in 1852 after accumulating between $3,000 and $4,000.