Abstract Background Physical inactivity is a widespread problem with a great need for innovative intervention concepts to overcome it. Epidemiological studies have identified working women in high-income Western countries to be at greater risk for physical inactivity. The current study included working mothers and examined within-subject associations between doing exercise/sport together with one’s child and five different affective states, and with light physical activity (LPA) and moderate-to-vigorous physical activity (MVPA). Method During 1 week, mothers (N = 192) completed up to eight ecological momentary assessment (EMA) surveys a day to assess momentary affect and certain situational circumstances (e.g., doing exercise/sport, being together with child). Physical activity was assessed objectively with waist-worn accelerometers. Results Multilevel analysis showed that doing exercise/sport together with one’s child was associated with higher positive affect and lower negative affect compared to being active alone. However, greater frequency of doing exercise/sport together with children was negatively associated with MVPA. Conclusion Due to the positive effect on momentary affect, combining spending time together with one’s child and simultaneously doing exercise/sport might be a good strategy of pairing two relevant personal goals. However, this strategy was not associated with sufficient MVPA.