Professionally, Pierre de Fermat was a lawyer; he had also contributed to physics.
Henry Poincare, Solomon Lefschetz, and Raoul Boot were into engineering when they were young.
Hugo Steinhaus was serious about applications, e.g. about the different feet shapes in order to help shoe designers. The famous Steinhaus slogan was
A mathematician will do it better.
In particular, Steinhaus patented longimeter.
Karol Borsuk invented a successful game during WWII, and it helped him to survive in those hardship years.
Stan (Stanisław) Ulam was the main inventor of the H bomb. He has also invented cellular automata.
Israel Gelfand at his mature stage turned his interests toward biology and medicine.
Rene Thom in his later years became seriously interested in too many things to mention here.
Several American mathematicians, including John Milnor, worked on DOD contracts.
Topologist James Munkres made a significant contribution to the assignment problem -- it's even called Munkres assignment algorithm or Kuhn–Munkres algorithm.
A parallel processor invented by a mathematician in the US, and the technology that followed, had an impact on the fall of communism.
Greg Kuperberg was an early pioneer in the field of computer games when he was in his teens; this helped him to pay his tuition at MIT.