Background The link between age at marriage and first birth in explaining completed family size is not always direct, due to heterogeneity in circumstances, that compel individual women to marry or initiate childbearing at a particular age. We analyzed data for 1020 women aged 45-49 in 2014 of the 1965-1969 birth cohort from the 2013-14 Zambia Demographic and Health Survey (ZDHS). Methods We fitted a bivariate and multivariate multinomial logistic regression to establish the relationship between a mother’s age at first marriage and at first birth on completed family size (CFS). Chi-square test of proportions measuring differences in proportions and relative risk ratios (RRR) with confidence intervals at 95% are reported. Results Our results show that the average CFS was 6.7 (95% CI: 6.5 – 6.9) among women completing their reproductive span in 2014 with mean age at first marriage and birth being 18.3 years (95% CI: 18.0 – 18.5) and 18.9 years (95% CI: 18.7 – 19.1) respectively. Women marrying at younger ages and having their first birth at younger ages were more likely (RRR: 1.187; 95% CI: 1.138-1.239 and RRR: 0.195; 95% CI: 0.074-0.511 respectively) to have higher CFS than their compatriots that initiated both marriage and childbearing at or after age 22 controlling for covariates as presented in the controlled model. The independent effects model shows that the risk of having 1-3 children compared to 6 or more children was lower (RRR: 0.073; 95% CI: 0.009-0.611; RRR: 0.136; 95% CI: 0,046-0.402 and RRR: 0.421; 05% CI: 0.135-1.312) for women whose ages at first marriage were <15, 15-18 and 19-21 respectively relative to 22+. Women with 1-3 children were 2.5 times more likely to use contraception than women with 6 or more children. Conclusion Having no education, being a rural resident and having a medium household wealth all increase the risk of having higher CFS. Women that marry before age 19 have a higher likelihood of having 6 or more children by the end of their reproductive period. The study concludes that apart from a woman’s age at first marriage and first birth, a complex network of factors interact to determined CFS.