Yamaha was founded in 1897 by Torakusu Yamaha to make Reed Organs, expanding to Pianos in 1900. It was originally called Nippon Gakki though the products were branded Yamaha and survived the war as a Piano maker. Starting in the 1950's it expanded its musical instrument range from just Pianos and Organs to pretty much everything and diversified into, sports equipment and motorcycles, (which it has since hived off into a separate business). It also officially changed its name from Nippon Gakki to Yamaha in 1987. 

Ukulele production started in 1955 with the model 80. In 1960 models 150 and 170 were introduced, (there was also a model 300 pineapple but I don't have the dates for that?), and in 1963 production ceased.  Yamaha only ever made Sopranos, and Ukuleles were never a major item in the catalogue. One notable thing about the early Ukuleles was, although mechanical friction and geared tuners were available and used by pretty much all of the competitors, the Yamaha Ukuleles had wooden tuning pegs. I believe this was changed in later models, though I have no firm information on this and the later ones I have seen may have just had the wooden pegs replaces, (? - I would do this if I owned one). Yamaha reintroduced Ukuleles in 2013 but only in Japan, with a range of premium, (from the price - They might be laminate I am unsure on the translators?), koa, (1000 series), and mahogany , (500 series) Soprano and Concert Ukuleles. 

I say proper Ukulele because in the intervening years Yamaha pretty much invented the Guitalele with its GL1. There were other Guitaleles available before this, I have seen old Hawaiian ones, but it is the GL1, which was launched in 1995, (and is still in production), as part of the Mini Guitars range, took the idea to a worldwide audience.