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)
*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