Nenghai was born Gong Xueguang in Hanwang Town of Mianzhu city, in Sichuan province, to Gong Changyi (), a peddler. He had an elder sister. When he was a child, both his parents died, leaving only him and his 10-year-old sister. By age 14, he became an apprentice in Hengshengtong (), and studied Classic and history under the proprietor. In 1905, he enrolled at the Army Academy (now Army War College), where he studied alongside Liu Xiang and Liu Wenhui. After graduating in 1907 he became a drillmaster at Yunnan Military Academy, both Zhu De and Yang Sen were his student. Then he served as regimental commander in Sichuan government, holding the position until he was transferred to the Beijing General's Office ().
