George was born in 1826 to Dr. George and Elizabeth McClellan. George's academic ability became apparent at a young age and in 1839 he was able to begin attending Pennsylvania State University. Despite studying to be a lawyer he suddenly changed his mind and in 1841 applied to attend the US Military Academy. In this position McClellan was responsible for supply and logistical activities. He remained in this post until 1847 when he was posted to the garrison at fort Middleton, Texas. During this posting McClellan befriended many southern officers, many future confederate generals.