First glass pH electrode was constructed in 1908 by Fritz Haber and Zygmunt Klemensiewicz. As the paper describing electrode was published year later (Z. Phys. Chem. 1909,. 67, 385.) it is usually assumed that the electrode was created in 1909. The original electrode had a glass bubble, was filled with strong electrolyte and there was an Ag/AgCl half cell inside, with Ag wire as a contact.
Today the general idea did not changed much. Potential difference builds up on the sides of thin glass in the bubble thanks to the difference between H+ activities on both sides, this potential difference is measured with the help of reference electrodes - and is known to be proportional to the pH on the outside of the bubble.
However, our practical knowledge about the pH electrodes is much wider now - and it allowed new designs, like combined electrode, double junction electrode or electrode with gelled electrolyte. At the same time thanks to the careful selection of glass todays pH electrode has much higher selectivity.
There are also completely new, solid state electrodes, not as popular yet.