He reached Fort Yukon in 1873, and managed a store with Jack McQuesten at the Fortymile River.
Harper formed a trading partnership with McQuesten and Captain Al Mayo; their company founded Fort Reliance in 1875 and other posts in the Yukon region.
Harper was known as the best prospector of the trio, and while he did not achieve major success in his pursuit of gold, he sometimes directed others to finds.
In 1875, Harper, McQuesten, and Mayo started a trading post that they called Fort Reliance on the Yukon River, supplying miners up and downriver.
[citation needed] After establishing their post, Harper and his partners all married young Athabascan Koyukon women.
[1] The Harpers had eight children together, six sons and two daughters: including Andrew, Fred, Sam, and Walter, the youngest boy.
[6] An educated man himself, Harper sent the seven older children to boarding schools, mostly in San Francisco, California, each beginning at age five.
[citation needed] After the couple separated permanently in 1895, Harper left for Dawson, and briefly remarried.
Walter became a mountain climber and guide, and in 1913 made the first ascent of Denali, with the expedition of Hudson Stuck.