The longest dinosaur as of current understanding, Supersaurus vivianae was diplodocid from the late Jurassic period. This behemoth comes from the Morrison formation from Colorado, in 1972 by Vivian Jones. Living approximately 153 to 145 million years ago, it was a whooping 40 meters (130ft) in length and 40 metric tons.

There is also a potential second species from Portugal, S. lourinhanensis, living around the same time. It is also one of the oldest dinosaur we have found, with results showing over 200 years old! While this is unlikely, the bones do show it being considerably mature. So old that it current calculations can not give reasonable ages. But, is definitely cool think about!
