Lilienthal held a doctorate from the University of Munich when Ludwig Philippson recommended him to head a school inspired by the Enlightenment in Riga, then a part of the Russian Empire.

As Pauline Wengeroff wrote in her memoirs decades later, "Dr. Lilienthal made it a point to gather many of Brest's young people around him every day, speaking to them of acquiring West European learning, offering useful bits of advice, sketching out their future as men of culture.

He won the hearts of these impressionable young people who, while remaining true to their parents' religion in matters of observance, were branching off into new paths in all other respects, turning even further from the cultural orientation of the older generation.

He wrote for several publications and was an advocate for both Jewish and secular schools, teaching at Hebrew Union College and serving on the Cincinnati board of education.

Lilienthal was later an active supporter of the movement to abolish slavery in the United States, though a minority of American Jews, primarily those in the South, were themselves slaveholders and disagreed strongly with his position.