Assuming you're talking of W.A Mozart, Clavier
Mozart was known as a performer primarily for his Clavier playing, and that was indeed his first instrument (arguably beginning to play at the age of 3, though he certainly composed from behind it as early as 5).
His father Leopold was a violinist, and I suspect likely the one that made you think of the instrument, though Wolfgang certainly played many instruments including the violin.
Regarding comments on his appointment as assistant Concert Master I suspect that the term Concertmaster in the context given above is synonymous with the term Kapellmeister (more a director of music than a principal first violinist), rather than that of a modern Concert Master in an orchestra, especially as orchestral music then was very differently understood and executed in that time.
Edit: Further research has uncovered that there is in fact a difference between a Kapellmeister and a Concertmaster (or "Konzertmeister") in this context, with the latter being somewhat less senior. I could not, however, find anything to suggest that Mozart primarily played violin in this capacity. Seeing as he was offered a position as "Court organist and Concert Master" while he held this position in Salzburg it would seem instead that his primary performing responsibilities were likely on the organ or a keyboard instrument, as his Salzburg position would be the prominent thing on his resume at that point.