Upon returning to India, Roy joined the
Provincial Health Service. In addition to a private practice, he served as a nurse when necessary. He taught at the
Medical College and Hospital, Kolkata and later at the
Campbell Medical School (now
NRS Medical College) and the
Carmichael Medical College Roy believed that
swaraj (the call to action for India's freedom) would remain a dream unless the people were healthy and strong in mind and body. He made contributions to the organisation of medical education. He played an important role in the establishment of the Jadavpur T.B. Hospital, Chittaranjan Seva Sadan,
Kamala Nehru Memorial Hospital,
Victoria Institution (College), and
Chittaranjan Cancer Hospital. In 1926, the Chittaranjan Seva Sadan for women and children was opened by Roy. Roy was also
Mahatma Gandhi's personal doctor and friend. In 1925, Roy ran for elections from the
Barrackpore constituency as an independent candidate for the
Bengal Legislative Council and defeated the "Grand Old Man of Bengal",
Surendranath Banerjee. Though an independent, he voted with the
Swaraj Party (the Parliamentary wing of the
Congress party in the 1920s). As early as 1925, Roy tabled a resolution recommending a study of the causes of pollution in
Hooghly and suggested measures to prevent pollution in the future. Roy was elected to the
All India Congress Committee in 1928. Roy conducted
Civil Disobedience in Bengal in 1929 and prompted
Pandit Motilal Nehru to nominate him as a member of the
Congress Working Committee (CWC) in 1930. The CWC was declared an unlawful assembly and Roy along with other members of the committee were arrested on 26 August 1930 and detained at
Alipore Central Jail. During the
Dandi March in 1931, many members of the
Calcutta Corporation were imprisoned. Congress requested Roy to remain out of prison and discharge the duties of the Corporation. He served as the
Alderman of the Corporation from 1930 to 1931 and as the
Mayor of Calcutta from 1931 to 1933. Under him, the Corporation expanded schemes for free education, free medical aid, improved roads and lighting, and water supply. He set up a framework for dispensing grant-in-aid to hospitals and charitable
dispensaries. In 1942,
Rangoon fell to the Japanese bombing and caused an exodus from Calcutta fearing a Japanese invasion. Roy, then serving as the Vice-Chancellor of the
University of Calcutta, acquired air-raid shelters for schools and college students to have their classes in, and provided relief for students, teachers and employees alike. ==Chief Minister of West Bengal==