Timmerman began her career at Humanity in Action, where she developed educational programmes on human rights for future leaders and then rose to director. As Dutch director of international human rights organisation Human Rights Watch, she founded the Dutch office (2009 - 2017) and worked as a consultant in the non-profit sector. From 2020 to 2022, she was executive director for peace organisation PAX. Besides being a consultant, Timmerman is also a member of the Dutch Committee of Human Rights Watch and is working on the realisation of a Queer Museum. Timmerman's experience with human rights and dedication to civil society organisations she brings with her to Atria.