In September 1909 he was the subject of a large search when he flew solo over western Massachusetts and lost contact with those on the ground; Harmon landed safely and returned to city via car and train.

In October 1909 at the St. Louis Centennial balloon race Harmon again set a flight duration record of 48 hours 26 minutes as well as an altitude record of 24,000 feet in his gas balloon New York. Later in October he was a witness for a Wilbur Wright flight of 27 minutes (the longest at a government aerodrome at the time).

Harmon began 1910 by attending Aviation Week in Los Angeles, California, bringing a Curtiss aircraft and a balloon; while there he conducted a number of balloon flights with passengers. In March 1910 he attempted a balloon flight from San Antonio, Texas to New York, but was brought down early by weather. In July 1910 he set an American endurance record for airplane flight in Boston, staying aloft 3 hours 3 minutes 30 seconds; the record was broken in September. Later in July Harmon made an attempt to fly across the Long Island Sound, but crashed when his Farman III biplane lost power; he was not seriously injured. By August 1910 Harmon had added two French Bleriot monoplanes to his collection.

On August 20, 1910, Harmon became the first man to fly across the Long Island Sound, flying from Garden City, Long Island, New York to Greenwich, Connecticut. Harmon attempted to land on the property of his father-in-law E.C. Benedict but could not locate it in the fading light of the day, instead landing nearby. On landing he hit a patch of rough grass which broke the chassis of the airplane. For the flight, Harmon was awarded the Doubleday-Page Trophy. Harmon and Glenn Curtiss, 1910 In September 1910 Harmon won the Harvard Cup at the Boston Aviation Meet; he competed in a bomb-throwing target contest and other events as an amateur aviator.