In 1939 Simjian had the idea to build the Bankmatic Automated Teller Machine, probably his most famous invention. He registered 20 patents for it and was considered "prior art device" in subsequent fillings. He finally persuaded the City Bank of New York, today Citibank, to run a 6-month trial. The trial was however not continued — surprisingly not due to technical inefficiencies, but to lack of demand. It seems the only people using the machines were a small number of prostitutes and gamblers who didn't want to deal with tellers face to face, Simjian wrote. Hence Simjian missed out on not only the commercial success, but also the fame associated with inventing the ATM. This credit is often attributed to John Shepherd-Barron (who led the team behind the first electronic cash dispenser) and Donald Wetzel (who directed a $5 million project to build upon Shepherd-Barron's and other European inventions in the late 1960s).

Simjian gained commercial success during World War II with another invention, his Optical Range Estimation Trainer, a kind of simple flight simulator, made from mirrors, light sources and miniature airplanes, used to train US military pilots in estimating the speed and distance of airplanes. Simjian sold over 2000 of these devices. Today's successor of Reflectone (after a number of mergers and acquisitions), CAE, is still making its money with flight simulation and control technology.

Simjian founded several other companies in the following years and invented a number of very different devices and technologies, e. g. a teleprompter, medical ultrasound devices, a remote-controlled postage meter, a golf simulator and a meat tenderizer. He never ceased inventing in his laboratories in Fort Lauderdale. At the age of 92, he got his last patent on a process for improving the sound of wood for musical instruments, seven months before his death in 1997.