About 20,000 people were also driven out of their homes in the south and western parts of the country.
About 20,000 people were also driven out of their homes in the south and western parts of the country, the Disaster Management Centre said.
"The Southwest monsoon began with very intense rain," deputy minister for disaster management Dunesh Gankanda told reporters in Colombo.
"There are some areas where we are unable to reach, but relief operations are under way."
The DMC said the toll rose to 91 dead and another 110 missing as reports came in from areas which had been inaccessible earlier in the day.
Sri Lanka appealed to the international community including the United Nations and neighbouring countries for help as the numbers of dead and missing climbed.
Heavy rains in the southern and western parts of the country have destroyed hundreds of homes and cut off several roads.
"The (foreign) ministry will continue to monitor the flood situation and seek assistance as required in consultation with the Ministry of Disaster Management,” the government said in a statement.
It said Colombo had already sounded out the UN and its South Asian neighbours.
The latest flooding was the worst since May 2003 when 250 people were killed and 10,000 homes destroyed after a similarly powerful Southwest monsoon, officials said.
On Friday the military deployed thousands of troops to reach marooned villagers and the airforce carried out several rescue operations to pluck people from rooftops of flooded homes.
In the early hours of the day a mountainside collapsed on a women's hostel at a tea plantation at Neluwa in the island's south, killing at least seven women, police said.
Evacuation notices were issued to residents along the Kelani river, which flows to the Indian Ocean through the capital Colombo, amid fears that it was about to burst its banks.
"The residents who are living along the Kelani should evacuate to high ground within six hours," the DMC said.
DMC officials said the monsoon had been expected on Thursday night and ended a prolonged drought that had threatened agriculture as well as hydro power generation.
The rains filled the reservoirs used for hydro power, which had hit rock bottom, raising concerns of power shortage in June.
But officials said most reservoirs were now so full they were in danger of spilling over and causing dangers of flooding to people living downstream.