Familial liability or susceptibility has been shown to increase the risk for psychotic disorders, however, little is known about familial liability (FL) among patients with major psychiatric disorders in Nigeria. This study aimed to determine the rate of FL and factors influencing the age of onset among patients with schizophrenia and bipolar disorders. This was a cross-sectional survey of patients with either diagnosis of bipolar disorder or schizophrenia based on ICD-10 criteria. Data were collected through a pretested socio-demographic questionnaire, incorporating age, duration of illness, the onset of illness, and family history (FH) of mental illness. Data were presented as tables, charts and bivariate analysis was performed to determine the relationship between variables. The level of significance was set at P < 0.05.  Of the 235 participants, 66 (28.1%) had a FH of mental illness. The age of onset ranges from 12-70years with a mean of 30.1years (SD=11.1years) and majority 85 (36.2%), were within the age group 30-39 years and had developed the illness before the age of 30years (54.5%). The males were twice more likely to developed mental illness before the age of 30 compared with their female counterparts [OR=2.42, (CI = 1.40-4.25), (p=0.013)], likewise, the singles compared with the ever-married  [OR=4.24, (CI=2.45-7.34), (p=0.000)]. Although nearly a third of the participants had a FL to mental illness, there was no association between age of onset and FH of mental illness. However, the males and the singles were more likely to have developed mental illness at an earlier age compared with their counterparts.