Early life stress (ELS) impacts children and youth populations, leading to long-term mood and emotional consequences including depressive and anxiety disorders. A key neurotransmitter involved in emotion and mood regulation is serotonin, also known as 5-hydroxytryptamine (5-HT). Serotonin carrying neurons, also referred to as serotonergic neurons, innervate brain-wide regions that are involved in regulating mood and emotion, thus playing a role in regulating different behaviours. Alterations in serotonin levels during critical brain developmental periods are associated with long-term changes in behaviour - however, the mechanisms underlying this association remain unclear. To examine how early life stress (ELS) impacts brain-wide serotonin activity and consequent behaviours, we used a mouse model of chronic developmental stress. We implemented the limited bedding and nesting (LBN) model during the first postnatal week of life (postnatal days [PND] 2-9). Upon adulthood, we performed a standardized behavioural test battery to assess social, anxiety-like, passive-coping, and acute stress response behaviours. Using the Pet1-cre x Ai148 transgenic mouse line, we conducted in-vivo calcium imaging via fiber photometry in mice expressing GCaMP6f specifically in serotonin neurons in the dorsal raphe nucleus (DRN) to visualize neuronal activity during behavioural testing. Finally, we performed immunohistochemistry (IHC) on collected brain tissue and analyzed 5-HT populations in the DRN and markers of neuronal activation (cFos) in downstream serotonergic regions of interest. We found that ELS mice did not show heightened anxiety-like behaviour, did display increased social drive, and exhibited passive-coping behaviours relative to controls. Furthermore, differences in behaviours were observed in females and males. We also observed differences in Ca2+ activity of DRN-5HT neurons among control and ELS female and male mice. Finally, we found higher cFos density in downstream serotonergic regions of interest in ELS mice compared to controls.