Dr. Massimo Rinaldi invented the IME 84, which is arguably the first   fully-transistorized electronic desktop calculator (see photos below). When Dr. Rinaldi was not adding to his some 60 patents, or making corporate-level decisions, he was engaged in underwater photography, amateur radio and collecting antiques.

Source: Prabook.com (world biographical encyclopedia)

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*Other contenders for "first" are the Friden 130 (EC-130), the Sharp Compet (CS-10A), the Mathatronics Mathatron 4-24, and the WYLE Scientific (WS-01).

IME 84 Prototype (pre-production model)
Source: Mr. L. Beaugrand