Age at first marriage has a significant effect on birth interval. This study investigates the relationship between age at marriage to birth interval and its associated factors in Ethiopia. The objective of the study is to examine the estimated age at first marriage to first birth interval and its main associated factors by age at first marriage and place of residence in Ethiopia. Data from the Ethiopia Demographic Health Survey (EDHS) 2011 were used. The Cox proportional hazard model is used to determine the significant factors contributing towards age at marriage to first birth interval in Ethiopia. The findings from the Cox proportional hazard model indicate that age at first marriage, religion, region, educational level and work status are the most important significant covariates of ‘age at first marriage to first birth interval’ in Ethiopia. Age at first marriage of women and their socio-economic factors contribute to differences in first birth interval. On the contrary, in Muslim-dominated regions such as Somali, the time of transition to motherhood is early due to their religious belief.