It is not clear what law of conservation you are talking about. There are several.
James Joule
The law of Conservation of Mass was invented by a French chemist Antoine-Laurent Lavoisier.
Isaac Newton originated the concept of conservation of momentum. I would not describe this as an invention, so much as an observation about nature.
It was more of a discovery than an invention. And it was not a single person who discovered it. The law gradually evolved over time.
If by the law of conservation you mean the Law of Conservation of Matter, then it states that matter cant be created or destroyed.
It is not clear what law of conservation you are talking about. There are several.
Scientists call this law the law of conservation of matter
The law of conservation of energy was not invented, discovered, or promulgated by one single person. Rather, it gradually developed over time, as the understanding of energy improved, and more types of energy were understood to be interchangeable.
The 1st Law of thermodynamics is a restatement of the law of conservation of energy.
The law of conservation of mass, which states that in a closed system, mass is neither created nor destroyed, it can only change form. This means that in a chemical reaction that takes place in a closed system, the mass of the reactants equals the mass of the products.
The law of conservation of mass/matter states that mass/matter cannot be created or destroyed in chemical or physical changes.
If by the law of conservation you mean the Law of Conservation of Matter, then it states that matter cant be created or destroyed.