Seshan was appointed an apprentice administrator, as assistant collector, for a year at Coimbatore, as a trainee of the Academy of Administration in Delhi. He was first appointed sub-collector in Dindigul. He moved to the Secretariat for Rural Development in Madras and appointed director of programs and deputy secretary, where he managed a local administration programme for panchayats, from 1958 to 1962. In 1962, he was appointed the director of transport of Madras State. In 1964, he was appointed collector of Madurai district. After two and a half years, he went to study at Harvard University on Edward S. Mason Fellowship where he earned a master's degree in public administration in 1968. After return, in 1969, he was appointed as secretary to the Atomic Energy Commission. From 1972 to 1976, he served as joint secretary at the Department of Space. In 1976, he returned to Tamil Nadu and was appointed the state’s secretary of industries and of agriculture for brief period. After differences with the Chief Minister of Tamil Nadu, he resigned and moved to Delhi where he was appointed as a member of the Oil and Natural Gas Commission and was in charge of personnel. After two years, he served as additional secretary to the Ministry of Space from 1980 to 1985. Later he became secretary of the Ministry of Environment and Forests from 1985 to 1988. He opposed the Tehri dam and Sardar Sarovar dam on Narmada river during his stint but was overruled. He was later given additional position of secretary of Internal Security which he served until 1989. In 1988, he served secretary of the Ministry of Defence for ten months. He was appointed 18th Cabinet Secretary, the senior most position in the Indian civil service hierarchy, in 1989 and later served as a member of Planning Commission.
He was appointed the 10th Chief Election Commissioner and had served from 12 December 1990 to 11 December 1996. He became best known for his election reforms. He redefined the status and visibility of the Election Commission of India. He identified more than hundred electoral malpractices and reformed the election process. Some of reforms he implemented include enforcement of election code of conduct, Voter IDs for all eligible voters, limit on election candidates' expenditure, appointing election officials from states other than the one facing polls. He curbed several malpractices like bribing or intimidating voters, distribution of liquor during elections, use of government funds and machinery for campaigning, appealing to voters' caste or communal feelings, use of places of worship for campaigns, use of loudspeakers and high volume music without prior written permission. During the 1999 Indian general elections, due to his reforms, 1488 candidates were disqualified for three years for failing to submit their expenditure accounts. It was reported that he reviewed more than 40,000 expenditure accounts and disqualified 14,000 candidates for false information. In 1992, the Election Commission cancelled elections in Bihar and Punjab due to electoral issues.
He received the Ramon Magsaysay Award for government service in 1996.
Personal life
He was married to Jayalakshmi Seshan from 1959 until her death in March 2018. They did not have any children. He knew several languages including Tamil, Malayalam, Sanskrit, English, Hindi, Kannada, Marathi and Gujarati.
