He was appointed professor of materia medica in 1864, and in 1867 exchanged his chair with John Michael Maisch, taking that of practical pharmacy, on which branch he continued to lecture until his death.

Parrish's shop was located close to the University of Pennsylvania, and in his discussions with the professors there he became impressed with the belief that students from all parts of the United States were coming to Philadelphia to study medicine and were leaving without a knowledge of pharmacy.

Many of these medical practitioners were returning to small towns and villages where there were no apothecaries, and their want of knowledge of pharmacy was a loss to themselves and a disadvantage to their communities.

Thus in the fall of 1849, Parrish established a school of practical pharmacy at his shop whose curriculum was addressed especially to medical students.

In August 1872, he was appointed commissioner to the Indians with a view toward establishing peace, but he was attacked by malarial fever and died.