Analyzing a collaborative public management innovation in tax administration in the US and the Netherlands, this article investigates how differences in accountability systems affect senior public servants’ felt accountability, which itself appears critical to understanding why innovations evolve differently over time, even when sharing similar starting conditions. The article makes three key contributions. First, employing elite interviews, findings highlight how institutional factors critically shape public servants’ felt accountability, hereby expanding existing scholarship and its concentration on microlevel psychological determinants. Second, against expectations, findings illuminate that ministerial-based senior public servants may display a greater degree of autonomy in innovation design and implementation compared to agency-based officials. Third, the article demonstrates that the configuration of accountability systems can change significantly over time, thereby critically affecting the evolution of innovations. This underlines the relevance of taking a longitudinal research approach to analyzing senior public servants’ attitude to collaborative innovations.