Power Minister Satyendar Jain administered the oath of office to Saini, who was selected on the basis of the recommendation of a three-member Selection Committee.
As DERC was headless for few months, the regulatory body's work, including the process of new tariff determination in the national capital, was affected.
A senior official said the appointment follows permission granted by the Delhi High Court in its order on February 24 this year.
Saini, a 1981-batch Indian Revenue Service officer, will have to strike a balance between the three power distribution companies, which have been demanding an increase in power tariff, and the AAP government, which is against it.