The Northwest Metro, the first of four Sydney Metro projects, is a turn-up-and-go rail service with 13 stations between Tallawong and Chatswood.
Trains will run every four minutes in each direction at peak times on the Metro Northwest Line.
In the future, trains may run up to every two minutes.
Premier Gladys Berejiklian said thousands of commuters now have access to world-class metro rail.
“We have delivered the Northwest Metro on time and more than $1 billion under budget," Ms Berejiklian said.
Minister for Transport Andrew Constance said Sydney Metro is Australia’s first fully accessible railway which will deliver fast, safe and reliable travel.
“The Metro line is currently being extended from Chatswood to Bankstown and by 2024 Sydney will have 31 metro stations and a 66 kilometre standalone metro railway,” Mr Constance said.