Bernthsen first studied mathematics and natural sciences but later turned to chemistry, which he studied in Bonn and then in Heidelberg under Bunsen and Kopp. Uninterested in the work on gases being pursued there, Bernthsen thought of leaving Heidelberg, but remained to study with Kekulé and Victor Meyer.
Heinrich Bernthsen received honorary degrees from the Technische Hochschule in Berlin and the University of Heidelberg.
In 1877 Heinrich Bernthsen became lecturer-assistant to Kekulé, and two years later he gave his inaugural academic lecture. In 1883 he was appointed extraordinary professor.
Bernthsen left the University of Heidelberg in 1887 to join the Baden Aniline and Soda Factory (BASF) as head of the main laboratory; he later headed the patent division.
Heinrich Bernthsen’s chemical work was mainly industrial and dealt with dyes of the acridine and azine groups. He explained the composition of such substances as methylene blue and safranine. He also gave the correct composition of sodium hyposulfite. Although he was generally more interested in analysis than in synthesis, he developed technically feasible processes for producing indigo, rhodamine, and tolyl red. As a patent director, he secured patents for his indigo process, contact sulfuric acid, and lac dye. In the course of obtaining patents for his firm, Bernthsen became active in the movement for patent law to protect industrial chemical processes and to strengthen the chemical industry.
Heinrich August Bernthsen died on November 26, 1931, in Heidelberg, Baden-Wurttemberg, Germany.
On March 17, 1884, Heinrich Bernthsen married Maria Magdelene Haubenschmied, the daughter of a judge. The couple had three children.