Viswanathan was born in Mattancheri, Cochin on 4 November 1914 to an Ezhava family. He was educated at St. Thomas College, Trichur, Maharaja's College, Earnakulam and Law College, Trivandrum. He took his bachelor's degree in Law from the Madras University in 1938 and started his legal practice in Cochin the same year. Simultaneously, he took up the management of a High School there and also evinced great interest in trade union work. Viswanathan was one of the founders of a major Port-workers' Union by the name of Cochin Thuramukha Thozhilali Union.
