He was elected in 1962 with 51.8 % of the vote for president in the ninth round and took office on 11 May 1962. Due to a serious illness he could no longer perform his duties in 1964. This first stood in the Italian constitutional history the problem of who have to determine a permanent absence of the President, so that new elections for the presidency could be convened by the President of the Chamber of Deputies. Detailed provisions to that had been neither adopted by the Constituent Assembly, nor by the Parliament. According to the prevailing opinion of Italian constitutional law that the Parliament, however, must determine the prevention. The problem was solved by segnis resignation on 6 December 1964.
In 1964 he was awarded the Charlemagne Prize. The politician Mariotto Segni is his son.