The role I remembered him best for (before I got to see the old opisodes of DS) was "Jackson" in the film "Andromeda Strain", where he played one of two very different survivors of a mysterious illness. It was a good, paranoid, cynical sci-fi thriller of the times (1971.). Mitchell was funny as heck--- his character survived the epidemic because his body chemistry was messed up by drinking Sterno, which he called "squeeze." (I don't know if this was good science or not, but Michael Crichton, who wrote it, was a doctor at the time, so perhaps it was a plausible premise.)
According to IMDB, that was Mitchell's last named role (a major one in this film--- the last role seems to have been a bit part) and if he didn't steal the movie, it wasn't for lack of trying. I remember everyone in the movie theater chuckling whenever he said "squeeze." Sadly, he died shortly afterward in January 1972, just a couple of weeks before his 67th birthday. IMDB doesn't mention the cause.. He looked older than his years in AS, even given that his character was supposed to be in poor health to begin with. So he may have been sick in real life by then.