According to Jack Lynch, Ph.D., of Rutgers University, Samuel Johnson's Dictionary of the English Language (London, 1775) was the standard English dictionary for one hundred and fifty years, surpassed only by the Oxford English Dictionary (OED) at the beginning of the twentieth century; the OED's first fascicle (Volume A-B) was published in 1884. Therefore one of Johnson's Dictionary's contributions to the English language was to provide the first definitive authoritative source for English language lexicographical reference.
Johnson's was not the first dictionary of the English language but it was the first great dictionary of the English language. The fourth paragraph of Johnson's Preface to the First Edition tells part of Johnson's goal and likewise part of what Johnson's Dictionary contributed to the English language:
When I took the first survey of my undertaking, I found our speech copious without order, and energetick without rules: wherever I turned my view, there was perplexity to be disentangled, and confusion to be regulated; choice was to be made out of boundless variety, without any established principle of selection; adulterations were to be detected, without a settled test of purity; and modes of expression to be rejected or received, without the suffrages of any writers of classical reputation or acknowledged authority.
Since Johnson's work on the Dictionary was almost single-handed, London was the focus of his efforts. The above paragraph of his Preface tells us that, though the London dialect of English had long been the prestige dialect and the dialect of literature and government, there was not a codified standardization of English so varieties of pronunciations, usages, spellings, and rules of language order (called grammar) were widespread and numerous. Therefore a second contribution was that Johnson's undertaking distilled some order and standardization upon the variety still prevalent in the preferred prestige London dialect of the English language in the latter part of the eighteenth century.
According to Petri Liukkonen, Johnson followed the models of French and Italian dictionaries and chose to illustrate his 40,000 definitions with about 114,000 quotes from literature and every other academic field, thus addressing one of the concerns he expressed in his Preface: "[words were] without the suffrages of any writers of classical reputation or acknowledged authority." When the OED was later being complied by Dr. James Murray, who began the work, Johnson's formulation of illustrating with quotations was carried on, in fact, OED uses the earliest known quote for each usage. Therefore another contribution of Johnson's Dictionary was to have begun the chronicling of the usages and meanings along with changes in usage and meaning in the English language.