Despite measures put in place by the Nigerian government in controlling the high population growth rate witnessed in the country, population growth rate still remains high due to population momentum coupled with low age at first marriage of mothers being one of the most contributory factors. Reproductive health conditions of women in the Northern region of Nigeria is very poor due to low age at first marriage thereby leading to high fertility behaviour in terms of children ever born (CEB) in the region. This study made use of cross-sectional study design of secondary data extracted from the 2021 multiple cluster indicator survey (MCIS) on selected socio-demographic variables of children ever born (CEB) and age at first marriage in the region in examining the association and relationship between age at first marriage of mothers and fertility behaviour in terms of children ever born (CEB) in the region. Findings revealed that age at first marriage of mothers is associated with children ever born (CEB) in northern Nigeria with: χ2 = 117.163, p=0.000; χ2 = 46.703, p=0.000 and χ2 = 52.795, p=0.000for north central, north east and north western region respectively at 0.05 level of significance. Multivariate analysis also revealed relationship between educational level of mothers and fertility behaviour in the region with: Prob>F = 0.0000, R. Squared 0.045 at 5% level of significance. It was concluded that there is need for the Nigerian government to enforce the minimum age at marriage of 18 years recommended by the 1988 national population policy in northern Nigeria as this will help in reducing early marriage in the region. The government also need to address issues relating to women empowerment as this will go a long way in improving women autonomy in terms of financial autonomy there by reducing the tendency of girls been forced to early marriage because of financial benefits.