In 1860, Campbell formed a partnership with entrepreneurs Henry Turton and James Dunbar. They established a sugar-processing plant, Pioneer Mill Company. Dunbar sold out of the business early on. The partners also bought the Lahaina Sugar Company when it went bankrupt in 1863. With the boom in the sugar industry in the Kingdom of Hawaii and high international demand, Campbell became one of the wealthiest people in Lahaina. He used his profits to purchase land on the islands of Oahu, Maui and the Big Island of Hawaii.