In 1903, he began to practice law in New York City.
[citation needed] He was again a member of the State Assembly (New York Co., 26th D.) in 1913, and was one of the Managers (i.e. assemblymen who acted as prosecutors) at the impeachment trial of Governor William Sulzer.
[citation needed] Greenberg was declared elected to the 137th New York State Legislature, and sat through the whole regular session while his election was contested by Progressive Joseph Steinberg.
On March 27, 1914, the day before the annual adjournment sine die, Steinberg was seated in place of Greenberg.
His election was unsuccessfully contested by Republican Courtlandt Nicoll.