The Einstein of popular thought is the young Einstein. This is the intellectual rebel of 1905 who, in one year, laid out the special theory of relativity and E=mc2, postulated the light quantum and used Brownian motion to make the case for the reality of atoms. These achievements were made prior to Einstein holding an academic position. He was then still a patent examiner in the Bern patent office. The years that followed brought Einstein a succession of ever more prestigious academic appointments; and, in the mid 1910s, he delivered his masterpiece, the general theory of relativity.
In all this, there was a real sense that Einstein was ahead of his peers, leading the way. The special theory of relativity was absorbed into the mainstream of physics fairly quickly. The general theory of relativity was not quite so readily accommodated. This was in part due to its burdensome mathematical demands of the theory, at least relative to the standards of mathematical expertise then found among physicists. But the tide was flowing with Einstein. When the eclipse expeditions of 1919 vindicated Einstein's theory and he became a popular hero, critics risked being seen as unimaginative reactionaries.
Einstein's work on the light quantum did not fare so well. It was regarded by many as an odd aberration from an otherwise brilliant mind. Even in the early 1920s, it was doubted by Niels Bohr, who had a decade before developed the first quantum model of the atom.
By the end of the 1920s, however, another Einstein began to emerge. As the quantum theory enjoyed success after success, Einstein found himself unconvinced. He took on the role of critic, complaining that the new quantum theory, for all its virtues, could not be the final theory. This was Einstein's new place in the physics community for his final quarter century, ending with his death in 1955. He remained a revered figure. But he became increasingly isolated and marginalized, as he labored on his alternative theories with the help of a few assistants. In the years after his death, it became clear that Einstein's objection to quantum theory failed, but not, for the reasons articulated by his arch antagonist Niels Bohr.
The old Einstein is a recalcitrant Einstein, unwilling to swim with the new quantum tide that flooded over physics. We should not judge that harshly. No thinker can ever think purely new thoughts. We all sit at the junction of the old and the new. Einstein was one of the first of new physicists of the twentieth century. His discoveries and methods exercised a profound, defining influence on the development of twentieth century physics. However, there is also a strong sense in which he was one of the last of the nineteenth century physicists. Perhaps he was the greatest of them.