“I never thought of myself as a museum artist who’s doing work for the wall,” he said. “For me the book is the final result.” He assumes that most people who see his work at the museum won’t know who he is. “But if they have some connection to something they see,” he added, “and then they read the book, the more I’ll feel like the show was a success.”

With sketch pads in hand, Mr. McMahon covered momentous events in the civil rights struggle, spacecraft launchings, national political conventions and the Vatican, turning out line drawings for major magazines and newspapers. Many were later colored by watercolor or acrylic paints, and most rendered scenes in a heightened, energetic style. His goal, he said, was to step beyond what he considered the limitations of photography to “see around corners.”

Photographers capture a moment, he said, but he could combine moments, often hours apart, into a single picture and thereby convey, he believed, a larger truth. He might, for example, pluck images from a political convention — a balloon drop, a speaker, a network camera — that never appeared together, and put them in the same frame.

Of Mr. McMahon’s nearly 9,000 pictures, perhaps the most dramatic was created in 1955 in Mississippi at the trial of the killers of Emmett Till, a black 14-year-old from Chicago who was accused of flirting with the young white wife of a grocery owner. Abducted from the home of an uncle he was visiting, he was beaten and shot, and his mutilated body was found in the Tallahatchie River, weighted with a cotton gin fan tied around his neck with barbed wire. The killing was a catalyst of the civil rights movement.

Life magazine hired Mr. McMahon to make courtroom sketches of the trial, held in Sumner, Miss., after the judge barred photographers. Mr. McMahon used a small spiral notebook, assuming most onlookers would think he was a print reporter doodling. He later redrew the sketches in his hotel room, and again in his studio.

One of the most celebrated of the trial sketches published in Life captured Mose Wright, Emmett’s uncle, as he stood and identified the two men on trial as the ones who had abducted the youth.

“I was grasping for a viewpoint that I could make the center of everything, and after he did that, I had just what I needed,” Mr. McMahon said. “He shook off 300 years of history to stand up and point like that.”

(A black photographer, Ernest C. Withers, who became renowned for his images of the civil rights struggle and who had sneaked a camera into the courtroom, also snapped a picture of the moment. His photo was published in The Chicago Defender, an African-American newspaper, and came to be regarded as an emblem of courage.)

A jury of white men acquitted the defendants. But in 1956, in an interview with Look magazine for which they were paid, they admitted guilt, knowing they could not be retried under laws against double jeopardy.

William Franklin McMahon was born in Chicago on Sept. 9, 1921. He sold a cartoon to Collier’s magazine while he was still in high school. After graduating, he enlisted in the Army Air Forces and served as a navigator on a B-17 bomber. His plane was shot down, and he was captured by the Germans. In prison camp, he drew his guards surreptitiously when he could find paper.

He returned home to marry his high school sweetheart, the former Irene Mary Leahy, and lived for many years in Lake Forest, Ill. She died in 1997. He is survived by his sons, William, Franklin, Mark, Patrick, Hugh and Michael; his daughters, Mary McMahon Taplin, Deborah McMahon Osterholtz, Margot McMahon and Michelle McMahon-Kubota; 13 grandchildren; and five great-grandchildren.

Margot McMahon said that her father’s wartime experience prompted him to abandon his plans to be a cartoonist. He used the G.I. Bill to study at the Chicago Academy of Fine Arts and other art schools.

After working for Life at the Till trial, Mr. McMahon, a freelance artist, covered almost every national political convention from 1960 to 2004, the Selma-to-Montgomery civil rights march, the Nixon-Kennedy debates and the Second Vatican Council.

He made several films using his pictures, and the showing of one, about Chicago at Christmas, became a tradition in the city.

Mr. McMahon insisted he was not a courtroom artist, although he was widely praised for his coverage of the Chicago Eight (later the Chicago Seven) trial of demonstrators who had been arrested during the 1968 Democratic National Convention in Chicago. He also said he was not an illustrator, although he was inducted into the Society of Illustrators Hall of Fame. He was definitely not a portraitist, he said, because he never met his subjects. “I sit in the corner and make drawings of them,” he said. And he even rejected the label of artist, though his work has been shown at many museums, including the Smithsonian. What he was, he said, was simply a reporter, who used art to tell stories.