Born in Berlin, Germany in 1907, the daughter of German-Jewish professor and Soviet spy Rene Kuczynski. Brother Jurgen and sister Bridgitte also became Soviet spies.
Became a Communist in 1924 when she became a member of the Communist Youth Movement. Became the head of the German Communist Party's Propaganda Section.
Moved to the United States with her father and brother who were engaged in espionage activities for the GRU. Returned to Germany in 1929 and married Rudolph Hamburger, a friend from her childhood.
In 1930, was instructed by Soviet Intelligence to move to Shanghai, China. Her husband, also a Soviet spy was already in Shanghai, under the guise of an architect. Ruth was more important to the GRU than her husband as she operated a major spy ring in China.
Became close friends with Agnes Smedley, an American journalist who would ultimately introduce Kuczynski to Soviet agent Richard Sorge. Ruth began an affair with Sorge, often allowing him to use her apartment as a meeting place. Established a cover as a journalist writing for pro-Communist newspapers.
Was ordered back to Moscow for advanced training in 1933. Returned to China six months later, under a new cover as a bookseller. Her actual task was to develop a strong relationship between the GRU and Chines Communists in Manchuria who were fighting against the Japanese.
Worked with a GRU agent whom she knew only as Ernst. Rumored to have engaged in an affair with him and gave birth to a daughter in 1935, believed to be Ernst's child.
Sent to Peking (now Beijing) in 1935. Chinese intelligence, with the help of Morris "Two Gun" Cohen, did a sweep of suspected spies, arresting Sorge's replacement. Ruth and her husband escaped with her two daughters. They returned to London and visited her parents (her father was now teaching economics at the London School of Economics).
Was joined in England by Olga "Ollo" Muth, her former nanny from Germany. Muth became a nanny for Ruth's newborn daughter Nina. Muth, at this point, was unaware of the couple's espionage activities. Accompanied her husband to Poland where Rudolph would serve as Senior GRU officer.
Was ordered back to Moscow for further training in June 1937. Was also awarded the Order of the Red Banner by the Soviet Union for her espionage activities and then order to Switzerland in 1938 to establish a new spy ring. Stopped first in England to meet with prospective agents, one of whom was Alexander Foote. Foote was further assessed by Brigitte Kuczynski. Foote joined her in Montreuz, Switzerland in 1938 to serve as a radio operator. Moving in with her.
Began operating under the code-name "Sonia." Merged her burgeoning network with the Lucy spy ring operated by Alexander Rado. Welcomed a new member into her spy ring named Leon Beurton. Began a relationship with Beurton immediately, ending the one with Foote.
Began denouncing the Soviet Union and the principles of Communism after Russia signed a non-aggression pact with Nazi Germany in 1939. Was actually acting on orders from the GRU in order to develop a guise for a deep cover operation planned for her. The GRU wanted to her to live as a British citizen, and thus requested that she marry Foote. Instead she married Beurton in February 1940 (she divorced Hamburger in late 1939). Obtained a British passport soon thereafter and prepared to move to England.
Did not plan to take Ollo with them to England. Ollo, distraught over the prospect of being separated from the children and angry at Sonia and Beurton, informed British authorities of their espionage activities but no one took much note of her claims and failed to follow up on them.
Moved to Liverpool, England in February 1941 and then to Oxford and prepared for her new espionage activities. Was joined by Beurton in the summer of 1942 but he was soon drafted into the British Army. Was assigned to oversee the activities of Klaus Fuchs, the atomic bomb researcher who had provided so much information during his work on the Manhattan project. Fuchs had originally been recruited into the Communist party by Brigitte Kuczynski. Her father had provided aid to her at several points during her activities, as had her brother, who would eventually be made a Lieutenant Colonel in the United States Army and in a great position to pass information to her.
Was placed under suspicion when he contacts with Fuchs came to light after his arrest. Was also linked to Sir Roger Hollis, former head of MI5, with whom Sonia had become acquainted in Switzerland and China. Speculation held that Sonia had actually recruited Hollis into Soviet control but he vehemently denied even knowing her. Was questioned along with her husband by British agents in 1947 regarding their alleged involvement in espionage activities. Both refused to answer any questions and no further investigation was evident.
Fled to East Germany with her children in 1950 and was joined by Beurton in one year later. Received her second Order of the Red Banner award in 1969 as well as the Order of Karl Marx in 1984. Wrote several books including her autobiography in 1977. Considered by many to be the greatest female spy ever.