Franklin McMahon "covered the world and its newsmakers with a pencil and a sketch pad," according to his Chicago Tribune obituary. Born in Chicago, McMahon attended Fenwick High School in Oak Park and, by the time he graduated, had already sold cartoons to Collier's Weekly. After serving in the Army Air Corps in World War II and taking night classes at several Chicago-area art schools, he settled with his wife in Lake Forest to raise their family.

McMahon thought of himself as an artist-reporter; he drew from life rather than being an "after the fact" illustrator. He was hired by Life magazine to sketch courtroom events at the Mississippi trial of Emmett Till's suspected killers in 1955. Later, he was courtroom artist at the trial of the Chicago Seven after the 1968 Democratic Convention in Chicago. The hundreds of drawings from these two trials are now owned by the Chicago History Museum. McMahon documented other notable events such as manned U.S. space launchings in the 1960s and 1970s, the United Farm Workers protest in 1968, the 1973 Watergate hearings, the 1995 Million Man March, and Barack Obama's 2004 Democratic Convention speech. His drawings were usually done with a charcoal pencil and then colored later with acrylic watercolors in his home studio.