Samuel Miller received the first patent for a circular saw. British patent #1152 was awarded in 1777 for a sawing machine powered by a windmill.

Tabitha Babbitt, a Shaker from Harvard, Massachusetts, appears to have conceived of the idea independently here in the United States. Her circular saw was the first used in saw mills. She came up with the idea in 1810 when working at her spinning wheel. She was watching two men struggling with a pit saw that was used to rip logs into lumber and realized that half of their energy was wasted because the saw only cut on the forward stroke.  Babbitt discovered how a way to solve the problem using her spinning wheel.
She mounted a tin disk with notches along its circumference. Being pushed into the spinning disk cut the wood with much less effort than a pit saw. Her idea was enlarged and adopted for use by the local saw mill.  Use of the circular saw became a common practice here. Because of her Shaker religion, Babbitt did not apply for a patent for her invention.