Katō, was born as Hattori Sokichi, the second son of a former samurai retainer of the Owari Tokugawa domain in Nagoya, Owari Province, in the town of Saya, Ama District in what is now part of the city of Aisai, Aichi Prefecture. He was adopted by Katō Bunhei at the age of 13.
He attended Tokyo Imperial University, from which he graduated at the top of his class from the Law Department, specializing in English common law.
After graduation, he worked as an employee of Mitsubishi zaibatsu, and was sent to London for two years. On his return to Japan in 1885, he became an assistant manager at the Mitsubishi head office in Marunouchi, Tokyo.
In 1887 he became private secretary to Count Okuma, . minister of state for foreign affairs. Subsequently he served as director of a bureau in the finance department, and from 1894 to 1899 he represented his country at the court of St James. He received the portfolio of foreign affairs in the fourth Ito cabinet (1900 - 1901), which remained in office only a few months. He then remained without office until 1908, when he again accepted the post of ambassador in London.
Appointed again to the same position in the Saionji cabinet (1906), he resigned after a brief interval, being opposed to the nationalization of the private railways, which measure the cabinet approved.
He became president of the conservative Kenseikai political party in 1916, whose policies he greatly influenced with his opposition to the genrō, support of the constitution and support for extension of popular suffrage.
He earned the reputation of being one of the strongest men among the junior statesmen.
In 1886, he married Haruji, the eldest daughter of Iwasaki Yatarō, the president of Mitsubishi.
