After moving his family to Denver, he was named president of the Denver Chamber of Commerce, and was influential in helping to build the Denver Municipal Auditorium in time for the 1908 Democratic Convention. He was a founder and president of the Denver Art League - a precursor to the Denver Art Museum. Campion was a co-founder of the Denver Museum of Natural History (now the Museum of Nature & Science) and his donated gold collection is on display at the museum. He also gave money, along with James J Brown, Dennis Sheedy and others, toward the purchase of land for the construction of the Cathedral of the Immaculate Conception in Denver, at Colfax Ave. and Logan St. The bell in the east tower is dedicated to his memory.
Campion's Capitol Hill mansion at 800 Logan St. was one of the city's showplaces, and sat north of what is now the Colorado Governor's Mansion, which was the former home of Charles Boettcher's son, Claude K. The Campion house was destroyed in 1963 and replaced with an apartment highrise. Campion's name is listed on the Donor's Wall in Denver's Civic Center Park at Colfax Ave. and Broadway.
During the Leadville Colorado, Miners' Strike of 1896-97, Campion hired labor spies to infiltrate the Cloud City Miners' Union, Local 33 of the Western Federation of Miners. Spy reports compiled by the Thiel Detective Agency and forwarded to John Campion are currently housed at the Colorado Historical Society.