After the demise of King Vimaladharmasuriya, his cousin Senarath ascended the throne and Kusumasana Devi became his chief consort too. King Vimaladharmasuriya himself nominated Senarath as the heir since his sons were too young and not suitable to shoulder the burden of kingship. The first part of his reign was full of political unrest. The eldest son of King Vimaladharmasuriya died soon after King Senarath decame the king and people suspected that it was his doing. And there occurred Rebels against the king. But he managed to make the atmosphere peaceful by 1620. Meanwhile, rebels occurred against Portuguese in Sabaragamuwa and Sathkorale in 1616, and the king supported them too. But king Senarath’s aim was not to expel Portuguese from the country, but to live in peace with them. Thus, a treaty was signed between the king and the Portuguese in 1617. By this treaty, Portuguese accepted King Senarath as the legal ruler of Udarata, while the King accepted Portuguese as the legal rulers of maritime region. The territories were also clearly defined under this treaty.
But this peaceful situation was disturbed by 1620. The king acted against Portuguese and a war was declared in 1629. Portuguese attacked Udarata and King attacked Jaffna, Sathara koralaya and Portuguese fortress in the eastern coast. In 1630, Portuguesecaptain Constantine de Saa assaulted Udarata and a battle broke between him and prince Maha Astane , the youngest son of king Senarath at Randenigala in Vellavaya. Portuguese met with a disgraceful defeat at this battle.
During the reign of king Senarath, the Kandyan kingdom was expanded to Sath korale, Satharakorale, Thun korale, Sabaragamuwa areas to the interior of the country from Trincomalee harbor and the areas under the Vanniya to the east and the south east of the kingdom. In 1629, kking Senarath divided the kingdom into 3 parts namely Matale, Uva and Senakadagala and entrusted them to his 3 sons Vijayapala, Kumarasinghe and Rajasinghe respectively.