While Samuel Johnson had other contributions, he is most well-known for publishing his dictionary A Dictionary of the English Language in 1755. This dictionary is considered to be the first reliable English language dictionary.
What made the dictionary innovative was the use of literary quotations that put the words in context, drawing most notably on the works of Shakespeare, Milton and Dryden. Until this point that had not been done for dictionaries, which focused on definitions and etymology without looking at the use. It also changed how subsequent dictionaries would structure their entries.
While Johnson’s dictionary did not contribute anything to the English language in terms of new words or grammar per se, it was considered the eminent English language dictionary for almost 175 years (until the Oxford English Dictionary was completed in 1928) and was the premiere reference for spellings, which no doubt had an effect on standardising the spelling of the English language. The notable exception would be Noah Webster’s dictionaries, which became the basis of spelling in the US and why US English has many spelling differences in comparison with English from other countries (including the notable examples colour vs color (US), aluminium vs aluminum (US), etc.)