K.G. Ramanathan was born in Hyderabad in South India. He completed his B.A. and M.A. in mathematics at Osmania University and the University of Madras respectively before going to Princeton to earn his Ph.D; his advisor was Emil Artin. At Princeton, Ramanathan also worked with Hermann Weyl and Carl Siegel. Thereafter he returned to India to team up with K. Chandrasekharan at the Tata Institute of Fundamental Research (TIFR) at Colaba in 1951.

Ramanathan was married to Jayalakshmi Ramanathan. He had two sons Ananth, Mohan and four grandchildren Aparna, Kavitha, Anita and Nikil.

At TIFR, he built up the number theory group of young mathematicians from India. For several years, he took interest to study Ramanujan's unpublished and published work. He was a Editorial board member of Acta Arithmetica for over 30 years. He retired from TIFR in 1985.

His wrote a paper with Mathukumalli V. Subbarao giving him an Erdős number of 2.

Ramanathan was given numerous awards during his more than 30 years service at TIFR: Padma Bhushan, 1983; Shanti Swarup Bhatnagar Award, 1965; Fellow of Indian Academy of Sciences; Fellow of Indian National Science Academy; Honorary fellow of TIFR.