I don't know his biography very well but from what I've seen he was influential in the Revolutionary War for two years until he was wounded in battle, and then didn't do much until the early 19th century. Can anyone shed some light on why he is considered a Founding Father if he didn't really do much during the Revolutionary Era?
While he wasnt a "Central" actor if you really want to label it you can unequivocally say he was the last of the Founding generation to hold high office, and the last active participant in the Revolution and more importantly the Ratification debates to be President, and the last of the Virginia Dynasty.
Consider that while Monroe enlisted and fought in the Revolution and was already a leading figure in Virginia politics by the time of the Ratification debates. He was quite active in the state debate, and Virgina itself was a key state to get to accept the new Constitution, his being part of the group that accepted advocating for reform after the Constitution was ratified was a large part of the success of the Federalist cause.
He was also a US Senator in the 1790's and was appointed by Washington to be the US Minister to France.
All told Monroe's first government office to the Confederation Congress then was in 1783, whilst his successor as President Quincy Adams, only began his career in 1794 as Minister to the Netherlands.