Concerned about all the labor violence in his city, Carter Harrison decided to attend the Haymarket rally and to make sure his presence was visible. This was not difficult, since Harrison was a large and charismatic man who frequently wore the "slouch" hat visible in this photograph. Harrison was the first witness called by the defense, when testimony on the defendants' behalf began on August 2.

The mayor testified that he heard most of Spies's and Parsons's speeches, except for the time he left to confer with Inspector Bonfield in the Desplaines Street station. He conjectured that Spies's seeing him in the crowd may have caused the anarchist to temper his tone. Whether or not this was the case, Harrison was prepared to disperse the rally if he thought it was becoming dangerous, and this never happened. He said as much to Inspector Bonfield before he left. Harrison was about to go to sleep when he heard the explosion.

Harrison was related not only to Presidents William Henry Harrison and Benjamin Harrison, but also to Thomas Jefferson. The mayor grew up in Kentucky, where he was born in 1825. He attended Yale, and did not move to Chicago until 1855. He made a substantial amount of money in real estate and then became active in politics around 1870, serving two terms in Congress. During the railroad strike of 1877, Congressman Harrison urged businessmen to keep their offices and factories open to protect the great majority of workingmen who were well behaved from the mob of "idlers, thieves, and ruffians." Harrison was generally a friend of labor as well as a supporter of the eight-hour day. A Democrat, he was elected mayor in 1879 for the first of four successive two-year terms (he lost a race for the governorship in Republican Illinois in 1884).

In 1891 Harrison took over the Chicago Times, and he was elected mayor again in 1893, but his fifth term was cut short when he was fatally shot at the doorway of his home by Eugene Prendergast. Prendergast was a mentally unbalanced man who claimed he was denied an office in the city government. The assassin surrendered at the Desplaines Street station, but was soon transferred to the same cell in the Chicago Avenue station where Louis Lingg was once held.

The Haymarket resonances would continue, as the anarchists' attorney William Black participated in Predergast's defense, and Clarence Darrow, who worked for amnesty for Samuel Fielden, Michael Schwab, and Oscar Neebe, tried unsuccesfully to save Prendergast from the hangman.