 Soon after his move to London, in 1881, Bosanquet joined his half-brother Charles, and his friend and former classmate C.S. Loch, in their work with the Charity Organisation Society (COS). This led to a lifelong association with the COS—one with which Bosanquet was indelibly connected. He was a Member of Council of the COS from 1898 until his death, serving as Vice Chairman (1901–1915) and as Chairman (1916–1917). He also served on Administration and District Committees of the COS and was involved in the operation (and, from 1908–1912, served as Chairman of the Executive Council) of the COS-sponsored School of Sociology and Social Economics from 1903 until its incorporation into the London School of Economics in 1912. 