He attended the public schools and moved in 1841 to Jackson County, Iowa, where he held several local offices. Fenn moved to California in 1850 and engaged in mining and ranching, studied law, and was admitted to the bar in 1862 and commenced practice in that part of Washington Territory which became a part of the Territory of Idaho upon its organization in 1863.
Fenn attended public schools and moved to Jackson County, Iowa in 1841, where he held several local offices. He then relocated to California in 1850, pursuing careers in mining, ranching, and law, being admitted to the bar in 1862 and beginning his practice in what would become part of Idaho when it became a territory in 1863.