The role of mesenchymal stromal/stem cells (MSCs) in tumour development and progression remains a subject of debate. Previous studies have reported contradictory outcomes, possibly due to variations in experimental design and the use of xenograft models. Xenograft models limit interpretation and translation due to cross-species variability. To address these limitations, we employed an isogenic mouse model of spontaneous breast cancer (BC) to investigate the impact of murine MSCs on BC development and progression. Methods: MSCs isolated from FVB/N mouse adipose tissue (mASCs) were administered to female mice with palpable mammary tumours. Tumour volume and mass were assessed, and analysis of histopathological necrosis and gene expression was conducted on mammary (MT) and lung metastatic tumours (LT). Results: No change in MT mass and volume was observed between mASC-treated and control mice. However, mASC treatment led to increased necrosis in LT but not in MT. Immunohistochemistry revealed that mASC-treated mice had fewer CD163+ anti-inflammatory macrophages in the LT but not in the MT. Tgf-β3, vegfr1, and cd105 were observed and downregulated in both MT and LT in mASC-treated mice. The downregulation of cd36 and tgf-β3 contributes to pro-tumourigenic activities, whereas the downregulation of vegfr1 and cd105 is associated with an anti-tumour effect. In the mASC treatment group, all cytokines tested for, except IL-27, were elevated. Conclusion: This study suggests that mASCs are anti-tumourigenic in pulmonary metastatic BC. Our findings emphasize the importance of considering the tumour microenvironment and employing relevant animal models when investigating the impact of MSCs on tumour progression.