George Mason was an Anti-Federalist which by today's understanding would have made him a Republican back then.
There is some confusion over the use of the term Federalist since its meaning changed sharply over a very short period of time. The original "Federalists" were supporters of the ratification of the Constitution in the years between 1787 and 1790. Those who had strong objections to the new document were labeled the "Anti-Federalists".
Proponents of the Constitution took the name "Federalists." Many of those believed in bigger government as do both parties of today but the Democrats being better known for that. Those who opposed the Constitution actually wanted a more purely federal system (as with The Articles), or limited government but the Articles left us too far to the right near anarchy so it had to me adjusted toward a more centrist agreement. The opposition of the Constitution standing alone was more or less forced into taking the name "Anti-Federalists." The lack of a
Bill of Rights was one of the main arguments that Anti-Federalists used to try to convince the public to reject the Constitution but later just demanded the Bill of Rights as their agreement. If there was to be a government with more control then there had to be laws to protect citizens from that government so that we don't go back to the British ways of governance. That would be the thought of smaller government and more freedom for "We The People"... hence I would argue that this belief no longer really exists today other than with the Libertarian Party.
A political party at its core is labeled based on their trust, belief and support of a certain level of government. The level of total government control and no government is considered the opposite ends of the scale; Socialism, Marxism and Communism (on the left) and Fascism (on the right). The scale follows on the right with Conservatives and then Republicans; to the scale following on the left with Liberalism and then Progressivism before they reach the extremes of their ends. The middle held both sides in a central and/or moderate position within each party and have been called many things.
Federalists (Democrats) : As a rule are advocates of a strong central government. They were somewhat pessimistic about human nature and believed that the government must resist the passions of the general public. One of the government's prime functions was to maintain order. The Federalists tended to place their faith in the talents of smaller governing elite. Today, the Democrats of that time look like staunch conservatives because we have "progressed" so far to the left that they are not even recognizable in today's meaning. And the Republicans of that time look like the Blue Dog Democrats of today.
Anti-Federalist: Today, they are not what we would even remotely recognize. The closest party of resemblance would be the Libertarian Party. I do not believe the Republicans are even close to being anything like the Anti-Federalists of the past. Our entire political systems has moved further and further left on the scale - hence the confusion over what the parties believe and how they correlate to the meanings and actions of past parties.
Washington, Hamilton, Reed and Randolph were all independent from any party and did not like the idea of even having separate parties until the later they had to submit in order to have support in their ideas which turned into the democratic-republicans. However, Randolph never took hold to any yet his inclinations and beliefs lean to the republican idea of protecting ones self from the tyranny of a too powerful government.
In summation, our forefathers were mostly God fearing Democrats by the sheer nature of wanting in their beliefs on government control but much less compared to the tyranny of King George. At that time this was a good thing because it was still "limited" but left of The Articles (near Anarchy). 1801 marked the official beginning of the two party systems but George Mason was definitely on the side of protecting the people's rights from government tyranny.