Vice-President of the international Theosophical Society and President of the American Section, Cook was born in England on May 18, 1887, and joined the Theosophical Society in 1914. He became a naturalized American citizen, was elected to the National Board of Directors, and appointed as National Treasurer before serving as President of the American Section (1931-1945) and international Vice-President (1946-1960) under C. Jinarajadasa and Nilakanta Sri Ram.
On his inauguration as President of the American Section, Cook stated that he assumed the office with a "sense of responsibility [that] comes from an unshakeable realization that it involves a direct obligation to those Great Ones who, through our founders, were and still are the real leaders in our movement to help the world to an understanding of the Ancient Wisdom." Cook lived at the American headquarters in Wheaton, which he named "Olcott" in honor of the President-Founder and where he memorialized his son, whose early death had begun a spiritual quest that led him to the Theosophical Society, by the creation of the Aubrey Garden on its campus.
While serving in the American presidency, Cook continued his business career as Vice-President and Treasurer of the Diamond T Motor Truck Company of Chicago, driving the thirty-five mile trip to his Chicago manufacturing plant each day. After putting in a day's work there, he would make the return trip to Wheaton, spending his evenings and his weekends on Theosophical work, often late into the night.
Through the financially difficult years of the 1930s and early 1940s, Cook guided the Society and often personally supported it financially. Joy Mills recounts that, when she joined the American headquarters staff in 1942, sometimes funds were lacking to pay the staff members their monthly $38 compensation, so Cook paid it out of his own pocket. Cook's business acumen, efficiency, and inspiration were instrumental in bringing the American Section through three difficult patches: the slump that affected Theosophical work around the world after Krishnamurti closed the Order of the Star in the East in 1929, the years of the Great Depression, and World War II. He left the American Section debt-free, and his dedication inspired workers at the American headquarters and throughout the nation.
After Cook left the American Presidency, he and his first wife, who had grown apart from lack of shared interests, were divorced. They had a daughter, Myrtle, who eventually settled in Montana. Just before his appointment as International Vice-President, Cook married Etha Snodgrass, who had been National Secretary while he was National President. At Adyar, Cook was instrumental in establishing, in 1947 (the centennial of Annie Besant's birth), a fund whose income was to maintain the Adyar estate and Theosophical work (the Besant Centenary World Fund, now called the Adyar Besant Commemorative, or ABC, Fund). Cook was also the impulse behind the founding of the School of the Wisdom, inaugurated by President Jinarajadasa on Founders Day, November 17, 1949. After Etha died in India in 1949, Cook married Jocelyn Todd-Naylor, an English physician who had nursed Etha in her final illness. He returned to the United States in 1953 upon Josephine Ransom's appointment to the international vice-presidency (although she died shortly thereafter in a London car accident, being then succeed by James Perkins, Cook's successor in the American Presidency).
In 1955 Cook became one of the founding trustees and the first chairman of the Theosophical Investment Trust, which has become a major source of support for the Amer-ican Section. The Cooks returned to Chicago in 1959 and then moved to England, where he died on August 5, 1965. Sidney Cook's vision for the Theosophical Society, enunciated early in his American presidency was “Every member a server. Every lodge an influence. Every Section a power. Adyar a world blessing.”