Congress retains Himachal Lok Sabha seat
She defeated her nearest rival Jai Ram Thakur, a Bharatiya Janata Party (BJP) legislator and former cabinet minister.
Earlier, this seat was represented by Pratibha Singh's husband and Chief Minister Virbhadra Singh, who resigned following his election to the state assembly in December 2012.
The main contest was between the Congress and the BJP.
In a statement from Delhi, Virbhadra Singh said: "I want to thank all the voters for the landslide victory of the Congress."
He said the victory confirms that people have expressed faith and confidence in the programmes and policies of his government.
Expressing gratitude to voters of rain-ravaged Kinnaur district, the chief minister said: "I specially thank the voters of Kinnaur who overwhelmingly participated even after the natural calamity hitting the region."
He assured them that the government would reciprocate with the same spirit by ensuring speedy development works and undertaking massive relief and rehabilitation works.
Counting of votes began at 8 a.m.
Election official said Pratibha Singh, a second time MP, was leading from the first round.
Nearly 53 percent of the 11,24,786 electors exercised their franchise in the two phases of voting held June 23 and 27.
The Mandi parliamentary 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.