Background Time to age at first marriage of women is the duration of time until the age at which they started living with their first partner. Time to age at first marriage is widely considered a proxy indicator for the age at which women begin to be exposed to the risks inherent in sexual activity. The purpose is to model the determinant of time to age at first marriage among women in Ethiopia using Cox models with mixed effects. Methods The 2016 EDHS sample was selected using a two-stage cluster design. The data set in this study were obtained from the Demography and Health survey conducted in Ethiopia in 2016. In this study, we used Cox models with mixed effects. Results Of all 15,683 women aged 15–49, 11,405(72.72%) were married and the median & mean age at first marriage for women living in Ethiopia were 17 years and 18 years respectively. Conclusions Based on the AIC criterion Cox frailty survival model (log-normal frailty distribution) was selected among Cox models with mixed effects as a better fit for the dataset. Analyzing based on the selected model showed that residence of women, educational level of women, head occupation, work status of women & head educational level were the most significant factors and religion of women, access to media of women & wealth index of a household were not significant factors at 5% level of significance. The clustering effect was significant and hence there was heterogeneity among the regions on age at first marriage.