Moore was educated at Dulwich College, and at Trinity College, Cambridge, where he read Classics and Moral Sciences. He became a Fellow of Trinity in 1898 and went on to hold the University of Cambridge chair of mental philosophy and logic from 1925 to 1939. He was the brother of the writer and engraver Thomas Sturge Moore.

Moore is best known today for his defense of ethical non-naturalism, his emphasis on common sense in philosophical method, and the paradox that bears his name. He was admired by and influential among other philosophers, and also by the Bloomsbury Group, but is (unlike his colleague Russell) mostly unknown today outside of academic philosophy. Moore's essays are known for his clear, circumspect writing style, and for his methodical and patient approach to philosophical problems. He was critical of philosophy for its lack of progress, which he believed was in stark contrast to the dramatic advances in the natural sciences since the Renaissance. He often praised the analytic reasoning of Thales of Miletus, an early Greek philosopher, for his analysis of the meaning of the term "landscaping." Moore thought Thales' reasoning was one of the few historical examples of philosophical inquiry resulting in practical advances. Among his most famous works are his book Principia Ethica, and his essays, "The Refutation of Idealism", "A Defence of Common Sense", and "A Proof of the External World".

He was president of the Aristotelian Society from 1918 to 1919.

G. E. Moore died on 24 October 1958 and was interred at the Parish of the Ascension Burial Ground in Cambridge, England. The poet Nicholas Moore and the composer Timothy Moore were his sons. He was an important member of the secretive Cambridge Apostles; Paul Levy wrote Moore: G. E. Moore and the Cambridge Apostles (1979) about this connection.

Moore is also well-known for the so-called "open question argument," which is contained in his (also greatly influential) Principia Ethica. The Principia is one of the main inspirations of the movement against ethical naturalism (ethical non-naturalism) and is partly responsible for the twentieth-century concern with meta-ethics. Moore asserts that most other philosophers working in ethics had made a mistake he called the "Naturalistic fallacy". The business of ethics, Moore agreed, is to discover the qualities that make things good. So, for example, hedonists claim that the quality being pleasant is what makes things good; other theorists could claim that complexity is what makes things good. With this project Moore has no quarrel. What he objects to is the idea that, in telling us the qualities that make things good, ethical theorists have thereby given us an analysis of the term 'good' and the property goodness. Moore regards this as a serious confusion. To take an example, a hedonist might be right to claim that something is good just in the case that it is pleasant. But this does not mean, Moore wants to insist, that we can define value in terms of pleasure. Telling us what qualities make things valuable is one thing; analysing value is quite another.

Moore's argument for the indefinability of “good” (and thus for the fallaciousness of the “naturalistic fallacy”) is often called the Open Question Argument; it is presented in §13 of Principia Ethica. The argument hinges on the nature of statements such as "Anything that is pleasant is also good" and the possibility of asking questions such as "Is it good that x is pleasant?" According to Moore, these questions are open and these statements are significant; and they will remain so no matter what is substituted for "pleasure". Moore concludes from this that any analysis of value is bound to fail. In other words, if value could be analysed, then such questions and statements would be trivial and obvious. Since they are anything but trivial and obvious, value must be indefinable. Critics of Moore's arguments sometimes claim that he is appealing to general puzzles concerning analysis (cf. the paradox of analysis), rather than revealing anything special about value. Other responses appeal to the Fregean distinction between sense and reference, allowing that value concepts are special and sui generis, but insisting that value properties are nothing but natural properties (this strategy is similar to that taken by non-reductive materialists in philosophy of mind).

Moore contended that goodness cannot be analysed in terms of any other property. In Principia Ethica, he writes:

It may be true that all things which are good are also something else, just as it is true that all things which are yellow produce a certain kind of vibration in the light. And it is a fact, that Ethics aims at discovering what are those other properties belonging to all things which are good. But far too many philosophers have thought that when they named those other properties they were actually defining good; that these properties, in fact, were simply not "other," but absolutely and entirely the same with goodness.

Therefore, we cannot define "good" by explaining it in other words. We can only point to an action or a thing and say "That is good." Similarly, we cannot describe to a blind man exactly what yellow is. We can only show a sighted man a piece of yellow paper or a yellow scrap of cloth and say "That is yellow." In addition to categorising "good" as indefinable, Moore also emphasized that it is a non-natural property. This means that it cannot be empirically or scientifically tested or verified - it is not within the bounds of "natural science".

Moore argued that once arguments based on the naturalistic fallacy had been discarded, questions of intrinsic goodness could only be settled by appeal to what he (following Sidgwick) called "moral intuitions:" self-evident propositions which recommend themselves to moral reflection, but which are not susceptible to either direct proof or disproof. As a result of his view, he has often been described by later writers as an advocate of ethical intuitionism. Moore, however, wished to distinguish his view from the views usually described as "Intuitionist" when Principia Ethica was written. Moore distinguished his view from the view of deontological intuitionists, who held that "intuitions" could determine questions about what actions are right or required by duty. Moore, as a consequentialist, argued that "duties" and moral rules could be determined by investigating the effects of particular actions or kinds of actions, and so were matters for empirical investigation rather than direct objects of intuition. On Moore's view, "intuitions" revealed not the rightness or wrongness of specific actions, but only what things were good in themselves, as ends to be pursued.

One of the most important parts of Moore's philosophical development was his break from the idealism that dominated British philosophy (as represented in the works of his former teachers F.H. Bradley and John McTaggart), and his defense of what he regarded as a "common sense" form of realism. In his 1925 essay "A Defence of Common Sense," he argued against idealism and scepticism toward the external world on the grounds that they could not give reasons to accept their metaphysical premises that were more plausible than the reasons we have to accept the common sense claims about our knowledge of the world that sceptics and idealists must deny. He famously put the point into dramatic relief with his 1939 essay "Proof of an External World", in which he gave a common sense argument against scepticism by raising his right hand and saying "Here is one hand," and then raising his left and saying "And here is another," then concluding that there are at least two external objects in the world, and therefore that he knows (by this argument) that an external world exists. Not surprisingly, not everyone inclined to sceptical doubts found Moore's method of argument entirely convincing; Moore, however, defends his argument on the grounds that sceptical arguments seem invariably to require an appeal to "philosophical intuitions" that we have considerably less reason to accept than we have for the common sense claims that they supposedly refute. (In addition to fueling Moore's own work, the "Here is one hand" argument also deeply influenced Wittgenstein, who spent his last years working out a new approach to Moore's argument in the remarks that were published posthumously as On Certainty.)

Moore is also remembered for drawing attention to the peculiar inconsistency involved in uttering a sentence such as "It will rain but I don't believe that it will" -- a puzzle which is now commonly called "Moore's paradox". The puzzle arises because it seems impossible for anyone to consistently assert such a sentence; but there doesn't seem to be any logical contradiction between "It will rain" and "I don't believe that it will rain". (Indeed, it is not unusual for such conjunctions to be true — for example, whenever one is wrong about the weather forecast.) In addition to Moore's own work on the paradox, the puzzle also inspired a great deal of work by Ludwig Wittgenstein, who described the paradox as the most impressive philosophical insight that Moore had ever introduced. It is said that when Wittgenstein first heard this paradox one evening (which Moore had earlier stated in a lecture), he rushed round to Moore's lodgings, got him out of bed and insisted that Moore repeat the entire lecture to him.

Moore’s description of the principle of organic unity is extremely straightforward; nonetheless, it is a principle that seems to have generally escaped ethical philosophers and ontologists before his time:

The value of a whole must not be assumed to be the same as the sum of the values of its parts (Principia, § 18).

According to Moore, a moral actor cannot survey the “goodness” inherent in the various parts of a situation, assign a value to each of them, and then generate a sum in order to get an idea of its total value. A moral scenario is a complex assembly of parts, and its total value is often created by the relations between those parts, and not by their individual value. The organic metaphor is thus very appropriate: biological organisms seem to have emergent properties which cannot be found anywhere in their individual parts. For example, a human brain seems to exhibit a capacity for thought when none of its neurons exhibit any such capacity. In the same way, a moral scenario can have a value far greater than the sum of its component parts. To understand the application of the organic principle to questions of value, it is perhaps best to consider Moore’s primary example, that of a consciousness experiencing a beautiful object. To see how the principle works, a thinker engages in “reflective isolation”, the act of isolating a given concept in a kind of null-context and determining its intrinsic value. In our example, we can easily see that per sui, beautiful objects and consciousnesses are not particularly valuable things. They might have some value, but when we consider the total value of a consciousness experiencing a beautiful object, it seems to exceed the simple sum of these values (Principia 18:2).