Starting as an enlisted man over 25 years ago, Fong originally fought against the Fire Nation on the front lines where his "no fear" attitude won him high praises from his superiors. Fong worked his way up the ranks, eventually becoming an officer. He was different from the other officers, having a less formal education, but this set him apart and helped him on his path to becoming a general. Since General Fong started at such a low rank, the men under his command consider him one of their own and they are extremely loyal to him.
In 100 ASC, General Fong was given the responsibility of providing Aang and company an escort to the Earth Kingdom city of Omashu. Bent on stopping the War, during Aang's stay at his fortress he informed Aang that if he could just trigger the Avatar State he could defeat the Fire Lord and end the War immediately. After many failed attempts at triggering Aang's Avatar State, he decided that he would have to activate it using force by putting Aang in genuine danger. To this end, he directed his soldiers to attack the Avatar.
The soldiers' attack upon Aang did not succeed in triggering the Avatar State, so Fong tricked Aang into believing he had killed Katara, which drove Aang to enter his Avatar State. Afterwards, General Fong was knocked unconscious by Sokka after he stubbornly thought of more plans to put Aang into his Avatar State. After Team Avatar learned that the firebenders would be powerless on the Day of Black Sun and told the Earth King, it was later said that the Invasion of the Fire Nation would be launched from his fortress.
However, due to the fall of Ba Sing Se, General Fong was not an active participant in the Invasion on the Day of Black Sun but did send some of his troops he had available to assist the invasion force.