Dr. Joseph Leo McGahn (c. 1917 – December 24, 1999) was an American obstetrician and Democratic Party politician from New Jersey who served for six years in the New Jersey Senate from 1972 to 1978, where he was a key figure in bringing casino gambling to Atlantic City.
McGahn was born in Atlantic City. He attended St. Mary's College of Maryland, graduating summa cum laude in 1939 and was awarded his Doctor of Medicine degree in 1943 from the University of Pittsburgh School of Medicine. After graduating from medical school, he served as a general surgeon in the United States Army Medical Corps during World War II.
After returning from military service, he practiced internal medicine and surgery in Atlantic City. Starting in 1951, he began a specialty in obstetrics and gynecology.
McGahn served on the Absecon, New Jersey City Council and as its mayor. He won the Democratic primary for State Senate in 1971 in the district, in which registered Republican Party heavily outnumbered Democrats, and ran for office without the support of the local Democratic Party leadership. On Election Day, November 3, 1971, McGahn defeated incumbent Frank S. Farley, who had served a total of 34 years in the New Jersey Legislature, to win a seat in the New Jersey Senate, in a race in which McGahn focused on corruption, in the wake of federal investigation of the Republican Party machine in Atlantic City.
Shortly after taking office, McGahn was the cosponsor of a bill to bring casino gambling to New Jersey. He was described by The New York Times as the "principal architect" of legislation that brought casino gambling to Atlantic City. He also pushed for the creation of the New Jersey Casino Control Commission, considered the most rigorous casino enforcement agency at the time.