The son of a Paris doctor, my first love was music and my primary instrument was the violin...in fact I pursued a music career and only turned to mathematics at the age of 35.
I was elected to the Académie des Sciences, despite having written only one paper on formulas for the sum of the mth powers of the roots of an equation and the sum of the symmetric functions of the powers of such roots.
I only wrote three more papers, on topics such the problem of the knight's tour on the chess board, combinatorial ideas, and the theory of determinants.
Kronecker claims that my first paper led to the field of modern algebra (even though LaGrange is usually credit with my ideas!), while Muir suggests I was the founder of the theory of determinants (even though Leibniz is usually credited with this title!).
Back to music, my theories led the French Royal Academy of Sciences to reclassify music as "an art" and remove it from the division of mathematical sciences.
Mathematician James Tattersall recently summarized my life: "Apparently, among the mathematicians of [my] day [I] was considered a great musician and among the musicians a great mathematician."
A final frustration is that math historians seem unable to give me the correct forename, with published forenames including Charles Auguste, Abnit, C.A., and Alexandre-Theophile...so, which is correct?
Alexandre-Théophile Vandermonde (1735-1796)