Phua studied at Balestier Girls' Primary School, Raffles Girls' School and Hwa Chong Junior College. She graduated from the National University of Singapore with a Bachelor of Arts degree in English and earned a Master of Business Administration degree from Golden Gate University. In a career spanning two decades, she held management positions at Hewlett-Packard and the Wuthelam Group. She then founded a regional leadership training firm, the Centre of Effective Leadership.
Her son was diagnosed with autism at the age of three. As Phua consulted professionals and did research on how to help him, she thought about how she could help other autistic children. She subsequently founded WeCAN, a charity which helps caregivers of autistic people and offers early intervention programmes for autistic preschoolers.
In 2005, Phua left the corporate world to be a full-time special needs volunteer. She and her partners sold the Centre of Effective Leadership to Right Management, a Manpower Inc. subsidiary. Phua then became president of the Autism Resource Centre. She often wrote to newspapers and government agencies on issues affecting the special needs community.
Phua was co-founder and former acting principal of Pathlight School, the first special school for autistic children in Singapore. The school offers mainstream curriculum and life skills education to its students. A believer in helping autistic people realise their potential and integrate into society, Phua helped develop many of the school's programmes. These include employability skills training through a student-run café, specialised vocational training, and satellite classes where Pathlight students mix with mainstream students. In four years, enrolment increased tenfold and the school attracted media attention for its impact on students.
Phua joined the ruling PAP's Jalan Besar branch in 2004. The following year, she was appointed to the Feedback Supervisory Panel, which leads the government's Feedback Unit. In the 2006 general election, she was fielded as a PAP candidate in Jalan Besar GRC, which was contested by the Singapore Democratic Alliance. During the election campaign, she promised to make Singapore a more inclusive society by representing the disabled and special needs communities. The PAP team won Jalan Besar GRC with 69.26% of the vote.
In July 2006, the PAP formed a workgroup, headed by Phua, to explore initiatives to improve the financial security of disabled and special needs children. Phua also led a committee that drew up a five-year plan to improve services for special needs children. The PAP studied their proposals and later implemented some, such as a National non-profit Special Needs Trust Fund. Laws were also drafted to prohibit abuse of the mentally disabled and to allow parents to appoint someone to look after their special needs children after they die.
Phua criticised the PAP's "many helping hands" approach, where social services are provided by charities and overseen by the PAP. She said that special education should be led by the Ministry of Education (MOE), as it had more resources and expertise. Minister for Community Development, Youth and Sports Vivian Balakrishnan defended the policy, arguing that it reduced bureaucracy and left jobs to the right people. However, the MOE agreed to play a more active role in special education, with increased funding and teacher training. The MOE also introduced more satellite classes in mainstream schools and helped special schools set targets for learning outcomes, such as O-Level passes. In 2014 Phua also spoke out during the second reading of the Remote Gambling Bill, calling for a complete ban on remote gambling.
In April 2016, Phua caused widespread controversy when, during a debate on the Ministry of Home Affairs' budget in Parliament, she suggested the installation of fences at communal areas in Little India, like playgrounds, to prevent the congregation of foreign workers there and to allow residents to use the facilities away from them. "Congregations of such high density are walking time-bombs and public disorder incidents waiting to happen," she remarked. She apologised two days later.