Randhawa joined the Indian Civil Service in 1934, then served in various capacities at Saharanpur, Fyzabad, Almora, Allahabad, Agra, and Rai Barelli until 1945, when he became secretary of the Indian Council of Agricultural Research (ICAR) for a year.
He was associated with the ICAR through its initial years and made huge contributions to this pioneering organisation which was responsible for the Green Revolution in India.
In 1947 he was in charge of the entire function where Jawaharlal Nehru delivered his famous Tryst with destiny speech.
He then served the Government of India as advisor, Natural Resources Planning Commission from 1961 to 1964 and as the special secretary, Ministry of Food and Agriculture, Govt.
The library at Punjab Agricultural University is named in his honour and maintains a collection of his works and laboratory instruments used by him.
Apart from numerous papers on algae, he published many books on art, history, culture and agriculture.