Zhenhai boasts a long history. At the foot of the Hengshan Mountain in Xiaogang and the foot of the Sheshan Mountain in Shaxi, traces of human habitation in the Neolithic Age have been found. After the establishment of the State of Yue in the late Spring and Autumn Period, administrative jurisdiction began here. In the 25th year of Emperor Ying Zheng, King of Qin (222 BC), the Kuaiji Commandery was set up and the Gouzhang County was established. Since it's located in the east of the county, it's called East of Gouzhang, which lasted for 843 years. In the 4th year of Wude (621 AD) in the Tang Dynasty, it was divided into two counties named Yao and Yin. In the 8th year of Wude, the County of Yin was renamed as the Maoxian County, and it's name was changed to the East of Maoxian County. In the 4th year of Yuanhe in the Tang Dynasty (809 AD), the Wanghai Town was built at the mouth of the Yongjiang River in the East of Maoxian County, marking the beginning of administration over Zhenhai. In May of the 3rd year of Kaiping in the Later Liang Dynasty (909 AD), Qian Liu, King of Wuyue, visited Mingzhou and built a city in the Wanghai Town; in the 8th leap month, Qian Liu established the Wanghai County (as was recorded by the "Taiping Huanyu Record") as the county was located at the seaside, benefiting from fishing and salt. It was renamed Dinghai County soon afterwards. At that time, the county only governed four townships, namely the Qingquan, the Lingxu, the Chongqiu, and Jintang.
In the 10th year of the Xining in the Song Dynasty (Year 1077), three townships of Lingyan, Taiqiu, and Haiyan of the Yin County were transferred to Dinghai. In the first year of Yuanfeng (Year 1078), Jintang was transferred to Changguo, and Qingquan was divided into two townships, a east one and a west one. From then on, the county governed seven townships and was designated as a high-level densely populated county. The system in the Yuan Dynasty inherited that was established in the Song Dynasty. In the 20th year of Hongwu in the Ming Dynasty (Year 1387), Changguo County was abolished and changed into an administrative region in the border area. The original Changguo County (now the Zhoushan City) was subordinate to Dinghai County until the reign of Kangxi in the Qing Dynasty. In the 26th year of Kangxi in the Qing Dynasty (Year 1687), the Dinghai County was renamed as the Zhenhai County after it had been established for 778 years. In the following year, Dinghai County was established from the original Changguo. As of then, Zhoushan belonged to Zhenhai for 300 years. The establishment of the Zhenhai County remained unchanged until the period under the ROC.
After the establishment of the People's Republic of China, due to political and economic construction and other factors, the change of organizational system and the change of county boundaries were quite frequent. At the end of Year 1954, 10 townships in the northern part of Longshan were transferred to the Cixi County; five townships of Hetou and Changshi and so on in east Cixi were transferred to Zhenhai; The Tanxu Island at the mouth of the Hangzhou Bay was transferred to Shengsi. In 1958, the Wenxi Township of the Yuyao County was transferred to Zhenhai; at the end of the same year, the Zhenhai county was abolished and merged into the Ningbo City. In January 1963, the establishment of Zhenhai County was restored. In July 1985, it was merged into the Ningbo City again. In October of the same year, the county system was officially abolished and the Zhenhai District of the Ningbo City was established. The name of Zhenhai County was used lasting for 299 years. The county was established and finally abolished with 1,077 years in between.