Chief Nkoebe Nkoebe said the Queen mother collapsed at the church where she was attending a retreat for the Catholic order of Saint Cecilia. She had apparently complained that she was tired.
The 62-year-old Queen mother was taken to Mantsoenyane Catholic Hospital where she later died.
Her sudden death has shocked many Basotho as she recently attended a number of public functions including the inauguration of the Bafokeng King in Rustenburg and the birthday celebrations of King Mswati III in Swaziland, the chief said.
Queen Mamohato leaves two sons, King Letsie III, principal chief Bereng Seeiso Seeiso and two grandchildren.
President Thabo Mbeki on Sunday conveyed South Africa's condolences to King Letsie III of Lesotho on the death of the Queen Mother.
Mbeki's message said: "On behalf of the government and people of South Africa, please accept, Your Majesty, our sincere and heartfelt condolences during this time of bereavement.
"Our thoughts and prayers are with Your Majesty and your family, as well as the Government and People of the Kingdom of Lesotho during this time of mourning."