1774-1780, and was a student of Caspar Frederik Harsdorff, then director of the academy and Denmark's leading architect in the late 18th century and now referred to as “The Father of Danish Classicism”.

The academy, under Johannes Wiedewelt’s leadership, supported Lillie's request for a trade license as a cabinetmaker in Copenhagen.

The Chancellery awarded him all guild rights in 1779 because he had won the academy's large gold medallion.

That same year, on Harsdorff's recommendation, he was hired by the new director, Carsten Anker, as inspector and designer at The Royal Furniture Storehouse (Det kongelige Møbelmagasin), replacing Georg Roentgen from Neuwied.

His first large work was the decoration of the suite at the castle for the newly married Princess Louise Augusta and Christian Friedrich of Augustenborg in 1786.

In 1787 he was cited for neglect of duties as a teacher at the academy, and was refused a travel stipend, which should have been his due as recipient of the gold medallion eight years previously.

After a bankruptcy in 1798 he left Copenhagen and Denmark, and moved to Lübeck, where the second half of his career began.

Lillie became the director and a professor of architecture, perspective and geometry at the Freie Zeichenschule, Lübeck in 1804.