John Clark Sheehan (September 23, 1915 – March 21, 1992) was an American organic chemist whose work on synthetic penicillin led to tailor-made forms of the drug.

Dr. Sheehan's research on synthetic penicillin paved the way for the development of customized forms of the lifesaving antibiotic that target specific bacteria.

Over the four decades he worked at M.I.T., Sheehan came to hold over 30 patents, including the invention of ampicillin, a commonly used semi-synthetic penicillin that is taken orally rather than by injection.

Leo C. Sheehan left home at the age of fifteen and found work as a reporter in San Francisco.

Green played a role in stimulating John C. Sheehan's interest in science by giving him a microscope with an oil-immersion lens.

He also introduced Sheehan to the curator of a local museum and took his grandson to meetings where Green frequently met with others passionate about astronomy.

Joseph is a professor of psychology at the University of California and ran training programs for relieving speech defects with his wife, Vivian.

As a post-doctoral fellow, Sheehan collaborated with Bachmann on the commercially feasible production of RDX also known as cyclonite, which turned out to give the Allies a huge advantage at sea during World War II.

After the efficient completion of RDX synthesis, John Sheehan had the experience of applying organic chemistry to real-life problems.

However, he wanted to pursue the synthesis of penicillin, which was hard in an industrial setting driven by results given that many scientists of his time believed that it was impossible.

For three decades after the discovery of natural penicillin by Sir Alexander Fleming, the source of the antibiotic hardly changed.

For the nine years that he worked on penicillin synthesis, there were practically no competitors, leaving Sheehan on a lonely search for a way to synthesize the antibiotic.

Most young academic chemists chose not to undertake projects that they perceived to be painfully slow because they wanted to impress faculty tenure committees with many experiments and publications.

Even though many of his friends openly questioned his decision of getting involved with the drug, Sheehan was determined to work on the chemical synthesis of penicillin at M.I.T.