Soni, former Vice Chancellor of Dr Babasaheb Ambedkar Open University, took the oath of office and secrecy as the UPSC member, an official release said.
The oath was administered by UPSC chairman Prof David R Syiemlieh.
Born on February 17, 1965, Soni, a scholar of political science with specialisation in international relation studies, earned his doctorate in "Post-Cold War International Systemic Transition and Indo-US Relations" from Sardar Patel University.
He has been the youngest-ever Vice Chancellor in independent (India).
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He will hold the office for a term of six years from the date on which he takes the charge or until he attains the age of 65 years.
With his appointment, the strength of the UPSC has gone up to eight. The UPSC, headed by a chairman, can have a maximum of 10 members.
The Commission conducts civil services examination annually in three stages -- preliminary, main and interview -- to select officers for Indian Administrative Service (IAS), Indian Foreign Service (IFS) and Indian Police Service (IPS), among others.