When was the martian dichotomy first observed?
It looks like the Mariner 9 era.
Based on a review paper by Watters et al. (&quot;Hemispheres Apart: The Crustal Dichotomy on Mars&quot;):  &quot;The north-south asymmetry ... was clear from the first global image mosaics of Mars returned in the 1970s by Mariner 9 (Mutch et al., 1976) and the Viking Orbiters.&quot;  Other papers cite a 1973 paper by Hartmann (&quot;Martian cratering 4, Mariner 9 initial analysis of cratering chronology&quot;) as one of the first papers to describe it, but it’s still based on Mariner 9 images.
I suppose in hindsight one could look at earlier flyby imagery and see that there is a dichotomy, but with 23 images from the first flyby, that's hardly representative of the surface and not enough to draw such conclusions.  Similarly, one could probably look at Earth-based telescopic observations from before the Space Age and retrofit observations into a north-south dichotomy paradigm, but I don't think one could legitimately claim earlier discovery of it.
So, 1971-1972 (Mariner 9 entered orbit in late 1971).