H.P. Lovecraft was never popular during his lifetime and died virtually unknown in 1937. Was he the last sci-fi writer to die unknown, only to become famous in later years? If not, who was the last one?
Given sufficiently tenuous definitions of "unknown" and "influential", I can mention Philip K. Dick. He lived in relative poverty as a science fiction author, and died March 2nd 1982 which is before the first film adaptation of one of his novels was released. That was Blade Runner, on June 25th 1982.
Now clearly PKD was known and respected within the SF community, having already won the Hugo in 1963 for his 1962 novel, The Man in the High Castle and the John W. Campbell for Flow My Tears, the Policeman Said. But he was unknown outside that community and posthumously influential on American (and broader) popular culture and particularly mainstream movie culture, so I think he qualifies.