The by-elections are seen as a prestigious popularity test for the ruling Congress-Janata Dal-Secular coalition, as the outcome is expected to have a bearing on the state's political scene.
Congress' Sowmya Reddy secured 54,457 votes against her nearest rival from the BJP.
The victories with huge margins came as a Diwali gift and a shot in the arm for the nearly six-month-old alliance ahead of the 2019 Lok Sabha polls after facing frequent questions about its longevity.