Yu was trained as a nurse in university. She became a nurse at a Hospital Authority public hospital after graduation. In the 2014 Occupy protests, she volunteered as a paramedic at the protest site which opened the door for her to social activism. She also volunteered in many protest scenes, including the 2016 Mong Kok civil unrest. She was also transferred from Intensive Care Unit to the Hospital Authority head office and planned to emigrate to Japan. In the 2019 anti-extradition protests, Yu became active in social activism, volunteering as a paramedic. The Hospital Authority Employees Alliance was founded in the call for forming labour unions to launch a city-wide strike to pressure the government, in which Yu became the chairwoman. Following the first outbreak of the COVID-19 in Hong Kong in January 2020, the Hong Kong government announced several control measures, including decreases in flights and closures of several border ports. Health care experts and the HAEA called for stronger border control closures than what the government has announced; at that time, areas in mainland China adjacent to Hong Kong, such as Shenzhen, had significantly more cases of coronavirus than Hong Kong. The HAEA called for a full border closure with mainland China, demanding that the government prevent all non-Hong Kong residents from entering the city via the mainland. The union attracted more than 12,000 applications to join as of January 2020. Following Chief Executive Carrie Lam's announcement to close four borders to the mainland, rather than a full closure demanded by the HAEA, the union launched a labour strike for five days between 3 and 7 February 2020. Yu said that over 2,500 hospital workers participated in the strike on the first day, with about 60 to 70 percent of them being nurses. The strike was attended by about 6,000 workers each day over the five days. In response, Chief Executive Carrie Lam announced that four more borders to be closed but refused to fully close the border. She also called the medical workers who were on strike as "radical". In response to the government's decision not to fully close the border, Yu shifted the union's demands, asking for adequate protection measures for HA medical workers and for the HA to rule out penalising staff who participated in the strike. as well as On 7 February, the HAEA fell 2,000 votes short of the target it set to extend industrial action into middle of next week. In response, Yu announced an end to the five-day strike, though she said that ending the strike "doesn't mean are giving up" with their demands. Yu participated in the 2020 Hong Kong pro-democracy primaries and received 2,493 votes, winning against incumbent lawmaker Joseph Lee in the Health Services functional constituency.
