After graduating, he worked as a civil and design engineer for several years in South East Asia before returning to Australia.

[7] In 1992 with business partner Bob Sharpless, Sinnathamby bought a 2,860 hectare (approx 7067 acre) area of land in Ipswich, south-west of Brisbane, for $7.9 million.

[14] A$1.2 billion dual track major rail line and transit hub was established in 2013 and connected Greater Springfield to the wider South East Queensland area.

[18] Sinnathamby has donated more than A$[clarification needed]200 million of land and funds for vital social infrastructure, and established a program of scholarships within Greater Springfield to assist people with limited access to education.

Sinnathamby's work was acknowledged globally in 2010 with the emerging Greater Springfield being named the world's best master-planned community by the International Real Estate Federation (FIABCI).

[32] In 2019, Sinnathamby received an appointment as a Member (AM) of the Order of Australia (General Division) as part of the Queen's Birthday Honours List for his significant service to the community and the construction sector.