<p>Healthy birth intervals are crucial for maternal and child health, yet their determinants among working women remain understudied in South Asian contexts. This study examined factors predicting healthy birth spacing (33–72 months between first and second births) among working women in Sri Lanka, with attention to occupational context. Using an exploratory-sequential mixed-methods design, we conducted 15 in-depth interviews with working women to inform questionnaire development, followed by a cross-sectional survey of 351 working women from four occupation sectors across 36 workplaces in Western Province. Binary logistic regression with cluster adjustment using Generalized Estimating Equations (GEE) identified predictors while accounting for workplace clustering. Overall, 61.3% of participants maintained healthy birth intervals. After cluster adjustment, significant predictors included younger age at first birth (ages 20–24: OR = 74.61, <i>p</i> &lt; 0.001), current marriage (OR = 5.46, <i>p</i> = 0.038), and conscious career prioritization (OR = 4.50, <i>p</i> &lt; 0.001). The model demonstrated excellent discrimination (AUC = 0.854) and good calibration (Hosmer-Lemeshow <i>p</i> = 0.386). Women’s occupation sector showed a bivariate association (<i>p</i> = 0.034) but not an independent association in multivariable models. Qualitative findings revealed four key themes: temporal negotiation, family as both resources and pressure, workplace as context rather than determinant, and healthcare as baseline enabler, which contextualize these quantitative patterns. Healthy birth spacing among working women is strongly associated with age at first birth, marital stability, and strategic career-reproductive planning. These findings suggest that individual-level resources and planning capacity may be more influential than sectoral differences in contexts with universal healthcare access, supporting policy focus on empowering women’s reproductive decision-making alongside workplace protections.</p>