In 1898, Nielsen arrived in China and was assigned to work at the Missionary Church in
Dandong,
Liaoning Province providing nursing care at the West Street Clinic. Because of an epidemic at that time, she was sent to Dagushan (), a town lying to the southwest of Dandong, where she saw up to 100 patients per day. There were conflicts with the local residents over the missionaries' refusal to accept Chinese medicine and their religious beliefs. In 1902, Nielsen established an industrial boarding school and began teaching the following year, with three students. She established an admittance policy which rejected any student with
bound feet. The school taught girls embroidery skills and then sold the handicrafts produced abroad to fund the institution, as well as providing them with classes in reading,
hygiene and religion. Two of her first three students were orphans, whom she had adopted. Nielsen founded the Chongzheng Girls' Primary School () in 1908 and four years later she opened a center for homeless women. The Chongzheng Poverty Relief Center () trained women in textile and embroidery manufacture. It expanded to include unemployed fathers, teaching them farming skills. Recognizing that many of the women had children, which impacted their ability to work, in 1913 she established the first kindergartens in Manchuria. In the spring of 1920, she built eight homes on the north side of Gushan Mountain, in the
Qianshan Mountain Range, to provide for the care of elderly and disabled villagers. Over the next several years, acting as principal of the school, she expanded the curricula to include a middle school, high school and
normal school. Deciding that she would live permanently in China, Nielsen gave up her Danish citizenship and applied for Chinese citizenship in 1929, becoming nationalized in 1931 and adopting the Chinese characters
聂乐信 for her name. Along with her citizenship, Nielsen was allowed to purchase land, and she used funds received from Denmark to create a collective known as Nielsen's Family Village. By 1939, the collective housed more than 300 people; there were 417 students and 18 teachers working in the girls' school from all over Manchuria and North Korea; the poverty center had 370 people enrolled in assistance programs, and there were 175 houses built as part of the collective. Using her salary, she paid each worker seven dollars a month in addition to sharing with them the crop yields and livestock, as well as providing free schooling for their children. She also purchased property on
Ludao Island to give the workers a place where they could take holidays. In 1942, during the
Japanese invasion of Manchuria, they overtook the school, changing the name to Dagushan National University of Higher Education. At the end of the
Second Sino-Japanese War, the
Communist Revolution began and several missionaries left the area in 1946. In 1947, the Communists arrived in Dagushan and seized Nielsen's personal belongings, as well as the land and properties. She was placed under house arrest and at her trial was convicted of being a landlord. Her two Danish employees, Nanny Brostrøm and Astrid Poulsen were jailed, and her Chinese employees were sent to work camps, leaving the elderly tenants to fend for themselves. After fourteen days, the Danish women were released and returned to Denmark. Most of the Chinese employees died in the camps. Nielsen refused to leave, because she was a Chinese citizen and had a responsibility to care for the villagers. When the communist administrators took over the village, they seized the church school, closed the factory and poor relief agency, and allocated the land and houses to other workers. In 1949, the new regime promised religious freedom and the church building, four cows, an orchard and a pond were returned to Nielsen. She tried to reorganize the community and revive the church, but her hopes were short-lived when another wave of
denunciations was prompted by the government and she was branded as an imperialist. By 1950, Nielsen was the only Danish missionary remaining in China and was forced to live in a small basement room. A faithful helper, Wang Chengren, assisted her in selling milk to meet her production quotas. Neighbors assisted her as she aged and lost her sight, Wang was arrested in 1959 and charged with assisting Nielsen, and then in February 1960, she fell, breaking her arms. ==Death and legacy==