Yes, he was.
And while some of his reforms caught on and remain in use to this day, his approach to dictionaries generally did not. When I said dictionaries do not do that, the verb tense was significant.
Webster lived 200 years ago, and today prescriptivism is rejected by the overwhelming majority of linguists, because it's based on a fundamental misunderstanding of what language is. Prescriptivism is about as well supported as phrenology, and shares many of its discriminatory goals. It was something people made up, that turned out to not make any actual sense, and that was subsequently abandoned by almost everyone.
Webster can instruct people on how to correctly write "Webster Language". But he cannot instruct people on how to correctly write English. English is whatever its users say and write. No other sensible definition exists.
He introduced American spelling and American English.
By constrast the Oxford English Dictionary was created by lexicographers intent on mapping the usage of English across space and time, they created multiple entries for each form | usage of root words and added copious notes (in the full multi volume OED editions) regarding first usage, alternate spellings, regional changes, etc.
The OED is a descriptive dictionary.