When "M. constructus" was first described, Young noted that the chevron bones indicated an affinity with Diplodocidae, but was uncertain to its exact position. In 1958, Young assigned "Mamenchisaurus" to the Titanosauridae. With the description of "M. hochuanensis", Young and Zhao created the family Mamenchisauridae in 1972. In 1978, when no "Mamenchisaurus" skulls were known, Berman and McIntosh assigned the genus to diplodocidae based on diplodocid-like vertebral features such as the forked chevrons. In 1990, McIntosh assigned "Mamenchisaurus" to a subfamily Mamenchisaurinae, which was placed inside Diplodocidae.