Background: Growing evidence suggests that sitting is activated automatically on exposure to associated environments, yet no study has yet sought to identify in what ways sitting may be automatic. Methods: This study used data from a 12-month sitting-reduction intervention trial to explore discrete dimensions of sitting automaticity, and how these dimensions may be affected by an intervention. 194 office workers reported sitting automaticity at baseline, and 3, 6, 9 and 12 months after receiving one of two sitting-reduction intervention variants. Results: Principal component analysis extracted two automaticity components, corresponding to a lack of awareness and a lack of control. Scores on both automaticity scales decreased over time post-intervention, indicating that sitting became more mindful, though lack of awareness scores were consistently higher than lack of control scores. Conclusions: Attempts to break office workers' sitting habits should seek to enhance conscious awareness of alternatives to sitting and afford office workers a greater sense of control over whether they sit or stand.