Population of Onslow County. Last official estimated population of Onslow County was 179,563 (year 2014) [1]. This was 0.056% of total US population and 1.8% of total North Carolina state population. Area of Onslow County is 905.9 miÂ² (=2346.3 kmÂ²), in this year population density was 198.21 p/miÂ². If population growth rate would be same as in period 2010-2014 (+0.25%/yr), Onslow County population in 2018 would be 181,372 .