A 20-kilometer (12 mi) stretch of the north shore of the island is not part of Chongming County or Shanghai but are instead two pene-exclaves of Jiangsu, formed by the connection of Chongming to the formerly-separate island of Yonglongsha.
Present-day Chongming first developed as two separate shoals—Xisha and Dongsha—during the Wude Era (AD 618–626) of the Tang dynasty. These were initially about 70 kilometers (43 mi) apart and located south of the larger island of Dongbuzhou (t東布洲, s东布洲, pDōngbùzhōu), which came to form part of the peninsula of Qidong in Jiangsu. Xisha was also known as Gujunsha. By the first year of Shenlong (705), a town was established on Xisha which was known as "Chongming". The number, size, and shapes of the islands fluctuated repeatedly over time and with the floods of the Yangtze: In the 3rd year of Tiansheng (1025), a new island arose a little to the northwest of Dongsha which became known as Yaoliusha from the surnames of its early settlers. In the first year of Jiangzhong Jingguo (1101), Sansha rose for the third time, stabilizing 50 li northwest of Yaoliusha. During the 12th- and 13th-century Yuan dynasty, Yingqiansha, Matuosha, Zhangpusha, and around six other islands also formed. The early settlers of these islands were mostly fishermen and salt collectors, with their principal saltern at Tianci.
In the 14th year of Zhiyuan (1277) under Kublai Khan of the Yuan, Chongming Prefecture was organized within Yangzhou. Owing to the shifting sands of the islands, however, the county seat needed to be repeatedly moved. During the late Yuan and early Ming, the sparsely-inhabited shoals of the area began to fill with migrants from Pudong and other areas of Songjiang Prefecture. These immigrants introduced cotton cultivation, which spread widely prior to China's opening to international trade in the later Qing. There was small-scale textile production in the area, but most of the cotton was exported for use in Songjiang, Hangzhou, and other more developed areas of Jiangnan. In the 2nd year of Hongwu (1396) under the Ming, the prefecture was downgraded to a county but, shortly afterwards, a major subsidence—sinking the entirety of Dongsha and most of Xisha—provoked a mass emigration back to the mainland. Around the same time, however, Sansha expanded towards the north while Yaoliusha simultaneously eroded in the south while building up in the north, "migrating" it towards Sansha.
Chongming County was later placed in Suzhou and then Taicang Prefectures. Over the course of the Ming, numerous shoals arose in succession: Ma'ansha, Chen'ensha, Gaomingsha, Xiaoyinsha, Changsha, Yuanjiasha, Xiangsha, Gaotousha, and around 20 others. Over the two hundred years from the beginning of Jiajing (1522) under the Ming to the end of Kangxi (1722) under the Qing, these shoals frequently flooded or collapsed back into the river. Yaoliusha and Sansha eroded away; Xisha, Chenensha, Fanliansha, and others sank. In the west, Pingyangsha arose, then sank, then finally merged with nearby Yuanjiasha, Wujiasha, Xiangsha, and Nansha. Around the end of the Ming dynasty in 1644, these islands were merged all the way to Gaotousha in the west, forming a single mass 200 Chinese miles east to west by 40 north to south. Their unification with remaining nearby islands by 1681 effectively created present-day Chongming, although it remained much smaller at the time. Between the Shunzhi and Kangxi Eras (1644–1722), Rilongsha, Yongansha, Ping'ansha, Dongsansha, and around 25 others arose without sinking.
Chongming is about 81 kilometers (50 mi) running east and west and generally between 13 to 18 kilometers (8.1 to 11.2 mi) north to south, covering 1,267 square kilometers (489 sq mi). The island has an average elevation of 1.6–2.6 meters (5 ft 3 in–8 ft 6 in) above sea level, with the northwest and central areas somewhat higher than the southwest and eastern ones.
Owing to the continuous expansion of the island, there are two pene-exclaves of Jiangsu present on the north shore of Chongming Island. The island of Yonglongsha appeared in the north channel of the Yangtze most recently in 1937 and was stabilized and expanded by workers from Nantong in Jiangsu during the late 1960s. Administration of the island was divided between Nantong's cities of Haimen and Qidong. Farms and then townships were established in both areas. Qidong's community of Qilong was connected in 1972 and Haimen's Haiyong township joined separately in 1975. Haiyong is now around 9 km2 (3.5 sq mi) while Qilong now stretches along 20 kilometers (12 mi) of the north shore of the island. As it is usually only about 1.5 km (0.93 mi) wide, however, its area is only around 36.8 km2 (14.2 sq mi).
Chengqiao's 10-square-kilometer (3.9 sq mi) Chongming Industrial Zone was established in March of 1994 and promoted to municipal status in February of 1996. It lies west of the town center and north of Nanmen Port.
Chongming is also a center of Shanghai's shipbuilding and maritime equipment industries, whose main base is on nearby Changxing.
Annual festivals on Chongming include the Shanghai Chongming Forest Travel Festival, Chongming Hairy Crab Festival, the Mingzhu Lake Cup Fishing Competition, the Qianwei Autumn Ecological and Cultural Festival, the Shanghai Fishermen Fun Festival, and the Chongming Cook Stove Painting Festival. The Yu Sheng Cup, a national cricket fighting tournament, is held annually outside the Xilai Ranch in Lühua during the National Day holiday in early October, with a purse of 10 000 RMB.
The new bridges form part of the G40 (Hushan) Expressway connecting Shanghai and Xi'an. Chongming is also spanned by another 38 highways. The two major highways of the island run along its length, Beiyan Highway to the north and Chenhai Highway to the south. There are 41 bus lines in Chongming County, accessing over 98% of its 227 communities.
^Some folk etymologies trace it to chong’s more usual meaning of "worshipped" or "respected", claiming that the island's formation began with the rafts that floated downriver following Sun En's failed revolt around AD 400 against the Eastern Jin dynasty. It first took the name Suiming—"furtive and bright"—before receiving a more dignified name as it stabilized during the Tang dynasty.