Mehegan played solo piano for many years at the "Composer's Lounge" and "Ambassador Grill and Lounge" in New York City, as well as the "River Cafe" in Brooklyn, NY. He also played at numerous clubs in Southwest Connecticut, where he lived and raised a family.
John had many students, and was good friends with several great jazz musicians. He used to help Art Tatum navigate the subway uptown to the Harlem jazz clubs. The Mehegan children (Tara, Sean, and Eben) remember a constant stream of jazz legends visiting their home in Westport, CT. Leonard Bernstein, Dave Brubeck, and Gerry Mulligan, to name a few, were fellow residents of Connecticut; jam sessions were not unusual, with various members sitting in as the day unfolded.
He authored numerous books on jazz, including the Jazz Improvisation series, which sets out the basic principles of jazz. These are published in seventeen languages and sold around the world.
The American composer Leonard Bernstein dedicated a piano composition to Mehegan in his Four Anniversaries collection published in 1948. Marked agitato: scherzando, the piece, For Johnny Mehegan, commemorates Mehegan's birthday, June 6, 1920.