Sen was the older brother of the more famous Ashoke Kumar Sen (1899-1996), Union Law Minister and a noted Indian barrister. Another brother was Amiya Kumar Sen, the last man to see Rabindranath Tagore alive.
Sen was born in 1899, the elder or eldest son of a district magistrate. He was educated at Presidency College, Kolkata and at the University of London. He was awarded a gold medal in Mathematics at the latter. In 1921, Sen joined the Indian Civil Service, and served in various districts as an ICS officer and as a judge. In 1947, he was appointed Chief Secretary of West Bengal, the senior-most rank that an ICS officer could attain in any state in British India. He was still serving in that capacity when he was sent on deputation as chief election commissioner.
He remained CEC from 21 March 1950 until 19 December 1958.
While Chief Secretary, Sen and his colleagues had to deal with the aftermath of an immense influx of refugees from East Pakistan (now Bangladesh) and the outgoing flow of refugees from West Bengal. There were also problems of riots and atrocities against Muslims for the administration to suppresss.
A meeting was arranged between the Chief Secretaries of the two Bengals. Sukumar Sen of the ICS (Later India’s first Chief Election Commissioner) was the Chief Secretary of West Bengal at the time, and he traveled to Dacca, and came back with very bad news. He said that an enormous refugee influx was in the offing, whose number could go up to a million.
Historian Ramachandra Guha writes of Sukumar Sen in 2002:
on India's first general election in 1952 has historian 's take on the first CEC:Nehru's haste [in wanting India's first general election] was understandable, but it was viewed with some alarm by the man who had to make the election possible, a man who is an unsung hero of Indian democracy. It is a pity we know so little about Sukumar Sen. He left no memoirs and, it appears, no papers either.
It was perhaps the mathematician in Sen, which made him ask the prime minister to wait. No officer of State, certainly no Indian official, has ever had such a stupendous task placed in front of him. Consider, first of all, the size of the electorate: 176 million Indians aged 21 or more, of whom about 85 per cent could not read or write. Each voter had to be identified, named and registered. This registration of voters was merely the first step. For how did one design party symbols, ballot papers and ballot boxes for a mostly unlettered electorate? Then, polling stations had to be built and properly spaced out, and honest and efficient polling officers recruited. Voting has to be as transparent as possible, to allow for the fair play of the multiplicity of parties that would contest. Moreover, with the general election would take place elections to the State Assemblies. Working with Sukumar Sen in this regard were the election commissioners of the different provinces, also I.C.S. men.
Tinker and Walker write that Sukumar Sen was aided by two Regional Election Commissioners plus one Chief Election Officer for each state.
The ability of India's first political leaders to refrain from interfering with the machinery, as well as their decision to retain the Indian Civil Service (renaming it to the Indian Administrative Service and making some cosmetic changes) gave Sen and his colleagues the freedom to adapt the machinery used by the British in the first Indian elctions for the purposes of a general election.