Mamenchisaurus was named by Young (1954).

It was assigned to Sauropoda by Huene (1958); to Titanosauridae by Young (1954), Young (1958), Huene (1959) and Romer (1966); to Cetiosauridae by Tatarinov (1964) and Rozhdestvensky (1977); to Homalosauropodoidea by Zhao (1983); to Diplodocidae by Berman and McIntosh (1978), Charig (1980), Bonaparte (1986) and Carroll (1988); to Mamenchisaurinae by McIntosh (1990), McIntosh (1990) and Dong (1992); to Euhelopodinae by Steel (1970), Dong et al. (1983), Dong (1990) and Martin-Rolland (1999); to Euhelopodidae by Upchurch (1995), Upchurch (1995), Buffetaut et al. (1995), Hunt et al. (1995), Buffetaut et al. (1997), Buffetaut et al. (2002), Royo-Torres et al. (2006), Buffetaut et al. (2006) and Lü et al. (2006); to Omeisauridae by Wilson (2002) and Allain and Aquesbi (2008); to Eusauropoda by Upchurch et al. (2004), Ksepka and Norell (2010), Otero and Pol (2013) and Mocho et al. (2014); to Mamenchisauridae by Chao (1965), Young and Chao (1972), He (1984), He et al. (1988), Russell and Zheng (1994), He et al. (1996), Zhang and Chen (1996), McIntosh (1997), Dong (1997), Dong (1998), Ouyang and Ye (2002), Peng et al. (2005), Naish and Martill (2007), Lü et al. (2008), Suteethorn et al. (2010), Sekiya (2011), Li et al. (2011), Suteethorn et al. (2013), Xing et al. (2015), Ren et al. (2018), Tan et al. (2019), Mannion et al. (2019) and Maisch and Matzke (2019); and to Mamenchisauridae by Moore et al. (2023).