Population of Grundy County. Last official estimated population of Grundy County was 50,098 (year 2014)[1]. This was 0.016% of total US population and 0.4% of total Illinois state population. Area of Grundy County is 430.4 miÂ² (=1114.9 kmÂ²), in this year population density was 116.39 p/miÂ². If population growth rate would be same as in period 2010-2014 (+0.02%/yr), Grundy County population in 2017 would be 50,124 .