Henry Pettit was born in Philadelphia, the son of Robert and Laura (Ellmaker) Pettit. He attended the University of Pennsylvania in the Class of 1863, but left during his junior year, later obtaining an honorary M.S. from the University in 1877. From 1862 to 1874 he worked in the engineering department of the Pennsylvania Railroad, where he became acquainted with Joseph Wilson
. In 1873 pettit was sent to the Vienna Exposition by the U.S. Centennial Commission and returned to Philadelphia to play a vital role in the planning and construction of the Centennial Exposition held in Philadelphia in 1876. Pettit served as Chief of the Bureau of Installation for the Centennial and as engineer and architect with Joseph Wilson for the main building at the Exposition. After the Centennial closed, Pettit retained his interest in exposition buildings and was architect for the U.S. Commission to the Paris Exposition of 1878 and a member of the Advisory Committee for Pennsylvania at the Chicago Exposition of 1893. Aside from his advisory tasks, Pettit was also a member of Frank Furness's
office from 1876 to 1888.
Pettit served as curator for the American Philosophical Society from 1897 to 1900 and as a director of the Union League from 1877 to 1878. He became an associate member of the AIA in 1872.