The present technology relates to signal processing device and method, an imaging element, and an electronic device capable of reducing a rise of costs. A signal processing device according to the present technology includes a measurement unit that performs measurement of a length of a period from an input start of a signal to a change of a value of the signal a plurality of times, retains measured values obtained by the measurement performed the plurality of times, sets an initial value of the measurement on the basis of any one of a plurality of the retained measured values, and performs the measurement by using the initial value. The present technology is applicable to an electronic circuit such as a flip-flop circuit and an A/D conversion unit, an imaging element such as a CMOS image sensor, and an electronic device such as a digital still camera, for example.