Professionally, Pierre de Fermat was a lawyer.

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.

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.

A parallel processor invented by a mathematician in the US, and the technology that followed, had an impact on the fall of communism.