The family soon moved to Gallatin, Missouri, where Kelly's father started a hardware and farm implement business;[3] his salary as a principal was insufficient to raise children.

[4] Kelly attended grade and high school in Gallatin, and graduated as class president and valedictorian at age 16.

[5] During his school years, Kelly worked various jobs, such as delivering newspapers, driving cattle to pasture for local farmers, and serving as bookkeeper for his father's store.

To support himself, Kelly worked for the Missouri Geological Survey, which allowed him to board in a room above its headquarters.

This experience changed his desired career path, and he switched to a general science course upon his return to the Missouri School of Mines and Metallurgy.

During his time at Chicago, Kelly was an assistant to Professor Robert Andrews Millikan, who won the Nobel Prize in Physics in 1923.

[3] After earning his Ph.D., Kelly was offered a job as a research physicist in the engineering department of the Western Electric Company by Frank B. Jewett, who later became the first president of Bell Telephone Laboratories.

"[5] Western Electric's engineering department was separately incorporated as Bell Telephone Laboratories in 1925, and Kelly transferred to the new company, where he worked as a research physicist until 1928.

Kelly was in charge of all military work at the laboratories, and directed programs whose funding amounted to $175 million (equivalent to $3 billion in 2023) for the war period.

[9] Kelly was involved in the Tizard Mission, and met with Edward George Bowen to attain information about recent British improvements to the cavity magnetron.

[10][11] Kelly remarked: "Progress has been made in some fields of technology in a four‐year interval that, under normal conditions of peace, would have required from 10 to 20 years.

[12] He purposely made the group interdisciplinary, teaming chemists, electrical engineers, metallurgists, and technicians with the solid-state physicists.

[5] The group was led by Shockley, who with John Bardeen and Walter Brattain was awarded the Nobel Prize in Physics for the invention of the first operational transistor.

In addition, he was a director of the Prudential Insurance Company of America, Bausch & Lomb, Tung-Sol, and the Economic Club of New York.

[3] After his retirement, he became an adviser to the NASA administrator James E. Webb,[5] and acted as a consultant to IBM and Ingersoll Rand.

Kelly was survived by his son, his daughter (wife to lawyer Robert von Mehren), and ten grandchildren.