A lot of people have contributed to the development of science (which includes mathematics). I have read that Newton's popularity was powerful enough to dominate the wave-model. Similarly, do we have any scientist whose work was underestimated, was not accepted by the community and was left unnoticed during his/her lifetime, but when a popular scientist published the same thing, got a wide appreciation?
George Green springs to mind. Whilst he had a very interesting, and not unsuccessful life, it was only after his death and Lord Kelvin looking through his works on mathematical physics that he was appreciated for quite how significant he truly was.
Julius Robert Mayer is now known for enunciating in 1841 one of the original statements of the conservation of energy (First Law of Thermodynamics). But his achievements were overlooked, and priority for the discovery of the mechanical equivalence of heat was attributed to James Joule. According to Wikipedia:
He attempted suicide on 18 May 1850 and was committed to a mental institution. After he was released, he was a broken man and only timidly re-entered public life in 1860. However, in the meantime, his scientific fame had grown, and he received a late appreciation of his achievement, although perhaps at a stage where he was no longer able to enjoy it.
I can think of
Alfred Wegener, the discoverer of plate tectonics, who died completely ignored before his theory gained acceptance.
Ludwig Boltzmann, defender of statistical mechanics in Physics.
Giordano Bruno, who was burnt at the stake for defending that the Universe is infinite and not centered around the Earth.
Galileo Galilei, who was kept under house arrest for the same reason.
Gregor Mendel, discoverer of the laws of genetic inheritance, who died completely unknown.
Ninety years ago yesterday (Feb. 18), astronomer Clyde Tombaugh gathered the data that proved the existence of what would eventually be dubbed Pluto — but it wouldn't have been possible, astronomers have since realised, without the calculations of a mathematician whom history has forgotten.
That mathematician was Elizabeth Williams, who worked for astronomer Percival Lowell, who first theorised the existence of a ninth planet. Lowell died before his successor, Tombaugh, finally spotted the elusive Pluto, but both men relied on calculations that Williams made. But the math got lost in the discovery it enabled, and so did Williams. Even her Wikipedia page is quite concise and states:
Williams continued to work at Lowell Observatory after Lowell's death, moving from Boston to the observatory itself at Flagstaff in 1919. In 1922, Williams married another astronomer, George Hall Hamilton. She was then dismissed from her position at the observatory by Constance Lowell since it was considered inappropriate to employ a married woman. In 1935, William's husband died. She moved to New Hampshire and subsequently died in poverty.
James Clerk Maxwell for coming up with his eponymous equations. According to Freeman Dyson:
In the year 1865, James Clerk Maxwell published his paper 'a dynamical theory of the electromagnetic field' in The Philosophical Transactions of the Royal Society...we, with the benefit of hindsight can see that Maxwells paper was the most important event in the history of the physical sciences... but the importance of Maxwell's work was not obvious to his contemporaries.
For more than twenty years his work on electromagnetism was mostly ignored. Physicists found it hard to understand because his equations were complicated. Mathematicians found it hard to understand because he used physical language to explain them. It was regarded as an obscure speculation without much experimental evidence to support it.
Twenty years after publication would be 1885. He passed away in 1879.