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. 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. While in the Senate, he served as chairman of the Committee on Agriculture, Energy and the Environment. Party leadership at the Democratic convention in 1977 roundly supported
Steven P. Perskie, a Democratic member of the
New Jersey General Assembly, by a margin of 266 to 77. Both McGahn and Perskie had won election to the Legislature in 1971, despite the Republicans 4-1 edge in registration. Perskie won reelection with 29,151 votes, defeating McGahn, who received 28,149. ==After the legislature==