The fate of four candidates, including two independents, will be decided during the day.
Nearly 53 percent of the 11,24,786 electors exercised their franchise in two phase of voting held June 23 and 27.
The main contestants are Congress's Pratibha Singh, wife of Chief Minister Virbhadra Singh, and Bharatiya Janata Party (BJP) legislator and former cabinet minister Jai Ram Thakur.
The bypoll was necessitated after Virbhadra Singh relinquished the seat following his election to the state assembly in Dec 2012.
The Mandi constituency, which includes Kullu, Mandi and some areas of Chamba and Shimla districts, besides the tribal-dominated Kinnaur and Lahaul and Spiti, is one of the biggest in the country.