After graduating from Columbia, he studied law and was admitted to the bar and built up a lucrative practice.

[3] On September 15, 1826, Jacob Barker, Henry Eckford, and other leaders of Tammany Hall were indicted for allegedly committing millions of dollars in acts of fraud against banks, insurance companies, and private citizens, and Maxwell subsequently prosecuted them for "conspiracy to defraud.

Eckford, a famous shipbuilder and entrepreneur of the time, was not prosecuted again after the first trial and sought an apology and public statement of his innocence from Maxwell, but succeeded only in getting Maxwell to make a statement that Eckford had been duped by others into illegal acts.

[5] After his term as New York County District Attorney ended in 1829, he resumed his law practice for the next twenty years, occupying a prominent position in the New York bar,[6] along with his law partner, Ogden Hoffman.

[7] An ardent Whig, in 1849, Maxwell was appointed by President Zachary Taylor as Collector of the Port of New York and remained in office through the Fillmore Administration until 1853, when his term expired.