[5] As a young man, he clerked in a mercantile house in Baltimore, and after attending St. John's College for short period, he decided in 1813 to move to New Orleans and embark on a career as a merchant.
However, he was unsuccessful in business, and found that the southern climate taxed his health, and therefore decided to return to Maryland to enter the legal profession.
[2] He was also elected to represent Anne Arundel County in the Maryland House of Delegates, during a period of financial crisis in the state.
He was not reelected, and a subsequent candidacy for the State Senate also failed, leaving Johnson to return to his legal practice.
[2] Throughout his legal career, Johnson maintained an interest in the welfare of St. John's college, advocating for it before the legislature and the public.