Kiernan began work as a messenger boy with the Magnetic Telegraph Company, and later with Western Union.
In 1869, he opened his own financial news service, Kiernan Wall Street Financial News Bureau, using a stock ticker.
About 1880, he hired Charles Dow and Edward Jones as financial reporters.
Dow and Jones left in 1882 to found the company which would soon launch The Wall Street Journal, and Kiernan went into partnership with William P. Sullivan.
He died on November 29, 1893, of "heart failure and pneumonia".