K. Kumar was one of the first socio-political leaders to bring the message of Mahatma Gandhi and the spirit of nationalism to the region of Travancore. He was Chairman of the Parliamentary Committee of Travancore. He also played a leading role in various movements for freedom including the Salt Satyagraha of Kozhikode, the Boycott of foreign goods and pickets in Alleppey, and the Vaikom Satyagraha.
Inspired by Gandhiji, K. Kumar quit higher education and joined as one of the few full-time collaborators at the Kerala Congress from Trivandrum. He lived in Trivandrum. His speeches caused a stir among both the intellectuals and laypeople. Few voices were as eloquent and moving as his. In the 1920s, he became part of an effort to revitalize the political scene in Kerala and set the tone for national movement called ‘Swadesha Bhimani’ - a movement revived by him, though started by Vakkom Mooravi and Ramakrishna Pillai.
K. Kumar also became the publisher and editor-in-chief of the newspaper earlier looked after by Ramakrishna Pillai. This was a bold move that nearly froze the government. He wrote impactful articles in addition to his usual editorials. He also played a significant role in other influential nationalist newspapers of that era. For instance, 'Swarat' being run by A.K. Pillai, and 'Mahatma' being looked after by the Amsi brothers. Kumar chaired the Travancore Council Committee and also directed Gandhiji's Travancore Tours. He was also the President of the Travancore Congress Committee.
K. Kumar served on the All India Congress Committee (AICC) and the working committee of the TC-PCC/ KPCC (Kerala Pradesh Congress Committee), heading its Constructive Work Committee during crucial years of the freedom movement. Besides Mahatma Gandhi, he had close ties with Rajaji, Pandit Nehru, C.R. Das, and other prominent leaders of the nation. Kumar was one of the most energetic of our freedom fighters who actively participated in practically all agitations for Indian freedom. Most notable of these were the leadership of the Salt Satyagraha (in Kozhikode, Tellicherry and Cannanore), the Civil Disobedience or foreign cloth boycott and picketing at Alleppey and other areas, and the Temple Entry Movement and eradication of 'untouchability', the Vaikom Satyagraha, and the Nagpur Flag Satyagraha. His patriotism earned him at least 21 months of imprisonment.
The prolonged agitation at Alleppey and Trivandrum brought about mass conversions to the Gandhian ideology and Khadi. K. Kumar’s leadership of the Swadeshi Movement and Foreign Cloth Boycott at Alleppey also inspired many prominent, educated women to come and support the national movement. By his late thirties, Kumar turned his attention to Harijan Welfare, Sarvodaya Education, and Khadi. He toured the state, delivering lectures and establishing scores of schools. He also started a school for Harijans named ‘Kumbazha Pravarthi Pallikudam’, which later became a life-giving tributary to the present Government VHSS Elanthoor. He continued to undertake the promotion of Khadi as a life mission. Dr. G. Ramachandran, a Gandhian and former Chairman of the Khadi Commission, was emphatic to the freedom fighter K. Kumar when he stated: "Kumar was Khadi, and Khadi was Kumar.”