The pendulum clock did not immediately come to Galileo’s mind when he noticed, in his late teens, that regardless the size of the arc a lamp hanging from the ceiling of an Italian cathedral makes when it swings, the time it takes to make that arc is always the same. Always.
Galileo would utilize this observation in his astronomical work which dominated his life. It was only in his old age that it occurred to him to apply it to mechanical construction and the keeping of time.
The First Pendulum Clock Design
The first pendulum clock of any practical import wasn’t invented until the mid-seventeenth century when Dutchman Christiaan Huygens patented a design in which he used a pendulum in conjunction with a verge escapement. (An escapement is the mechanism in a clock movement that controls the rate at which the clock ticks. The verge escapement is also known as the crown-wheel escapement.)