Mamenchisaurus was first discovered in 1952 on a highway construction site in Sichuan, China. The partial skeleton fossil was then studied, and named in 1954, by the renowned Chinese paleontologist Professor C. C. Young.
The first specimen discovered (the type specimen) was 22 m (72 ft) long and nearly half of that was neck, which made it the longest known neck of any animal at the time. 19 vertebrae were discovered (another record), along with the cervical ribs.
In 1972, a second species of Mamenchisaurus was discovered (M. hochuanensis) with a neck that reached up to 9.5 m (31 ft) in length. In 1994, the Sauroposeidon was discovered in the United States, with a neck estimated to be between 10.5 and 11.5 meters (34.5–37.5 feet) long, though its neck did not exceed that of the previously known Supersaurus, with a neck reaching 13–14 meters (42.5–46 feet).
In 1993, M. sinocanadorum was described; this species possessed the longest cervical rib of any described sauropod dinosaur, measuring 4.1 m (13.5 ft). This is longer than the longest Sauroposeidon cervical rib, which measures 3.42 m (11.2 ft).