Firstly, its worth recalling what Asaf writes in the comments:
Yes, pioneers are often considered "ahead of their time", but we
forget that the reason we think of them as ahead of their time is that
their ideas were thoroughly developed later because they required
several decades to be processed by the mathematical community at
large.
That being said, I think that everyone is bound by the dominant paradigm in which they're surrounded; yet somehow, certain key individuals are able to break with this paradigm just enough to do the ground-breaking work that they're remembered for. When this happens, their insights often seem to "come out of the blue," and it sometimes takes the wider mathematical community many years to catch up. Here's a couple of examples that I find particularly compelling.
Jean Baptiste Joseph Fourier (1768 – 1830) was one of the first people to think of a function as, well, an arbitrary function, rather than an explicitly given rule; hence the functions $f : [0,1] \rightarrow \mathbb{R}$ and $g : [0,2] \rightarrow \mathbb{R}$ given by $x \mapsto x^2$ are actually different. This was necessary to develop what we now call the Fourier transform, the then-groundbreaking (frankly I think its still groundbreaking) idea that just about every function $I \rightarrow \mathbb{R}$ can be expressed as a superposition of sine and cosine waves, where $I$ is a real interval. Even today, the Fourier transform is among the most important of tools in the arsenal of many engineers, physicists, and applied mathematicians; and the realization that functions with different domains need to be distinguished was a crucial step toward the pure mathematics of today that we know and love.
Georg Cantor (1845 – 1918) is, of course, the grandfather of modern set theory. Among other things, Cantor invented the ordinals, proved that every set could be well-ordered; and, building on Fourier's idea of a function as-an-arbitrary function, Cantor was able to formulate the concept of two sets being equipotent. Cantor realized that both $\mathbb{Z}$ and $\mathbb{Q}$ are equipotent to $\mathbb{N}$, and later, much to his shock, that $\mathbb{R}$ is not! I think he also discovered the $\aleph$ indexing of the cardinal numbers. Writing in a mathematical climate that was fiercely constructive and deeply suspicious of infinity, Cantor's work was largely ignored, and many of the top mathematicians of the day openly ridiculed his ideas. Nonetheless, he kept on working on his set theory (which he believed had theological significance) until the very end of his days, especially the continuum hypothesis. Its sucks that Cantor's work wasn't recognized sooner; once we started paying attention, pure mathematics was changed forever.
There's a lot more people I wanted to talk about (especially Godel, Samuel Eilenberg, Saunders Mac Lane, Garret Birkhoff, Alexander Grothendieck, and William Lawvere) but maybe I'll just leave it there.