Aristotle (~300 BCE) may, in some sense be called the first physicist, authoring a book called Physics, in which observations were explained in terms of a unifying theory of causes. A rock falls because it is seeking its natural place, as does smoke when it rises. The notion of a force, as experienced by humans, was taken for granted and not defined. It was properly assigned (in some sense) as the cause of motion, but Aristotle's explanation of its exact relationship to motion is verifiably incorrect by modern methods (described below).
The fact that Aristotle said something verifiably incorrect is an important development. Many aspects of philosophy have no basis in observation. It is in proposing theories about testable relationships between concepts that Aristotle is a physicist. To be clear, no person is a isolated genius. There were other contemporaries participating in the birth of "natural philosophy" as it would be known until ~1700 CE.
The most fundamental sense in which the physics of Aristotle is lacking is that it is not mathematical.
Not much happened in terms of scientific thinking for roughly the next 2000 years. Chemical recipes were discovered and engineering marvels were created, but the conceptual framework for understanding the physical world did not shift again until the renaissance.
One of the most pivotal players (again, not alone) was Galileo (~1600 CE). He is known for many things, but his certainly most important contribution was to develop the first mathematical formulations of physical laws. In other words, he was the first to summarize observations using an algebraic formula, such as the equations students now find in their physics textbooks. Though lacking the notation of calculus, there was present the notion of acceleration (change in velocity over time). In his famous experiments of throwing balls off the leaning tower of Pisa, he correctly surmised that gravitational acceleration is independent of the mass of an object (as long as wind resistance can be neglected, such as for rocks). Galileo also refined the notion of inertia, paving the way for Newton's first law: a body in motion stays in motion. Forces are necessary to start and stop motion, but not maintain it, as Aristotle had asserted.