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Early life and education 
She was born Kathleen Rita McNulty in the small village of Creeslough in the Gaeltacht area (Irish-speaking region) of County Donegal, Ireland in 1921 during the Irish War of Independence. On the night of her birth, her father, James, who was an Irish Republican Army training officer, was arrested and imprisoned in Derry Gaol for 2 years. On his release, the family emigrated to the United States in October 1924 and settled in Pennsylvania where James McNulty established a successful stonemasonry business. At the time, Kathleen was unable to speak any English, only Gaelic; she would remember prayers in Gaelic for the rest of her life.
After attending parochial grade school in Chestnut Hill and Hallahan Catholic Girls High School in Philadelphia, she graduated with a degree in mathematics from Chestnut Hill College for Women in June 1942 (the attack on Pearl Harbor had shaken her senior year). Out of a class of 92 women, Kathleen was one of 3 math majors to graduate that year, and all of them had taken every mathematics course offered: two semesters of algebra, the history of math, integral calculus, spherical trigonometry, differential calculus, projective geometry, partial differential equations, and statistics. (In high school she had taken a year of algebra, a year of plane geometry, a second year of algebra, and a year of trigonometry and solid geometry.)
During her third year of college, Kathleen began the job hunt, knowing that she wanted to work in mathematics but did not want to be a schoolteacher. She learned that insurance companies' actuarial positions required a master's degree and were seldom filled by women anyway. Feeling that business training would make her more employable, she took as many business courses as her college schedule would permit: accounting, money and banking, business law, economics, and statistics.
Career as a computer programmer 
A week or two after graduating, she happened to see a U.S. Civil Service ad in The Philadelphia Inquirer. Under the headline "Wanted: Women With Degrees in Mathematics," it read, "The need for women engineers and scientists is growing both in industry and government... Women are being offered scientific and engineering jobs where formerly men were preferred. Now is the time to consider your job in science and engineering... you will find that the slogan there as elsewhere is 'WOMEN WANTED!'." The Army was looking for women with mathematics degrees—right there in Philadelphia. She immediately called her two co-math majors, Frances Bilas and Josephine Benson. The latter couldn't meet up with them, so Kathleen and Fran met in Philadelphia one morning in June 1942 for an interview in a building on South Broad Street (likely the Union League Building), where they were informed of positions available through Aberdeen Proving Grounds at the University of Pennsylvania. Shortly thereafter, the two both received letters telling them to report for a week at the Moore School of Electrical Engineering, University of Pennsylvania, 33rd and Walnut Streets, beginning a day in early July 1942. At a starting annual salary of $1,620, pay for the position computing ballistics trajectories used for artillery firing tables (mostly using mechanical desk calculators and extremely large sheets of columned paper) was low (S.P. 4, a "sub-professional" pay grade), but both Kathleen and Fran were satisfied to have attained employment that utilized their educations, during wartime, having had no prior employment experience, and that served the war effort. Her official civil service title, as printed on her employment documentation, was "computer." With about 10 other "girls" (as the female computers were called) and 4 men—a group recently brought to the Moore School from Aberdeen Proving Grounds in Maryland--Kay and Fran would conduct their work in a large, empty classroom on the first floor of the Moore School; the same room would later be the one where the ENIAC was built and operated until December 1946.
Despite all their coursework, their mathematics training had not prepared Kay (as she came to be called early on at the Moore School) and Fran for their work calculating trajectories for firing tables: they were both unfamiliar with numerical integration methods used to compute the trajectories, and the textbook lent to them to study from (Numerical Mathematical Analysis, 1st Edition by James B. Scarborough, Oxford University Press, 1930) provided little enlightenment. The two newcomers ultimately learned how to perform the steps of their calculations, accurate to ten decimal places, through practice and the advisement of a well-liked supervisor, Lila Todd. A total of about 75 young female computers were employed at the Moore School in this period, many of them taking courses from Adele Goldstine, Mary Mauchly, and Mildred Kramer. The work was tedious, and many of them dropped out due to workload, but Kay became prominent among the computing women.
After 2 or 3 months, Kay and Fran were moved to work on the differential analyzer in the basement of the Moore School, the largest and most sophisticated analog mechanical calculator of the time, of which there were only 3 in the United States and 5 or 6 in the world (all of the others were in Great Britain). Using the analyzer (invented by Vannevar Bush of MIT a decade prior and made more precise with improvements by the Moore School staff), a single trajectory computation—about 40 hours of work on a mechanical desk calculator—could be performed in about 50 minutes.
Career as an ENIAC programmer 
The ENIAC was developed for the purpose of performing these same calculations between 1943–1946. In June 1945, Kay was selected to be one of its first programmers, along with several other women from the computer corps: Betty Snyder, Marlyn Wescoff, and Ruth Lichterman, and a fifth computer named Helen Greenman (nicknamed "Greenie.") When Greenie declined to go to Aberdeen for training because she had a nice apartment in West Philadelphia and a 1st alternate refused to cut short a vacation in Missouri, Betty Jean Jennings, the 2nd alternate, got the job, and between June and August 1945 they received training at Aberdeen Proving Grounds in the IBM punched card equipment that was to be used as the I/O for the ENIAC. (Later, Kay's college schoolmate and fellow computer Fran Bilas would join the team of ENIAC programmers at the Moore School, though she did not attend the initial training at Aberdeen.) The computer could complete the same ballistics calculations described above in about 10 seconds, but it would often take one or two days to set the computer up for a new set of problems, via plugs and switches. It was the women's responsibility to determine the sequence of steps required to complete the calculations for each problem and set up the ENIAC according; early on, they consulted with ENIAC engineers such as Arthur Burks to determine how the ENIAC could be programmed.
Because the ENIAC was a classified project, the programmers were not at first allowed into the room to see the machine, but they were given access to blueprints from which to work out programs in an adjacent room. Programming the ENIAC involved discretizing the differential equations involved in a trajectory problem to the precision allowed by the ENIAC and calculating the route to the appropriate bank of electronics in parallel progression, with each instruction having to reach the correct location in time to within 1/5,000th of a second. Having devised a program on paper, the women were allowed into the ENIAC room to physically program the machine.
Much of the programming time of the ENIAC consisted of setting up and running test programs that assured its operators of the whole system's integrity—every vacuum tube, every electrical connection needed to be verified before running a problem.
Kay McNulty was transferred to Aberdeen Proving Ground's Ballistics Research Laboratory along with the ENIAC when it was moved there in mid-1947; she was joined by Ruth Lichterman and Fran Bilas, but the other three women began families or started other jobs, preferring to stay in Philadelphia rather than relocate to the remote Aberdeen and live an Army base life.
Family life 
ENIAC co-inventor John Mauchly, who had since departed his post as a professor at the Moore School to found his own computer company along with Presper Eckert, made frequent trips to Washington, D.C. during this period, and stopped in to check up on the ENIAC in Aberdeen. Mauchly had already hired Betty Jean Jennings (who had married and now went by Jean Bartik) and Betty Snyder (now called Betty Holberton) and had hoped to attract Kay to his fledgling company as well. But Mauchly's wife had died in a September 1946 drowning accident, and as a recent widower with two children, Mauchly instead proposed to Kay, who was almost 14 years his junior.
Resigning her post at Aberdeen, and without the blessing of her Irish Catholic parents, she married him in 1948. They lived initially in his row house on St. Mark's Street near the University of Pennsylvania, and later in a large farmhouse called Little Linden in Ambler, Pennsylvania. With Mauchly, Kay had five children.
Later life 
John Mauchly died in 1980 following several bouts of illness and recoveries, and she married photographer Severo Antonelli in 1985. After a long struggle with Parkinson's disease, her second husband died in 1996; Kay had suffered a heart attack while caring for him, but made a full recovery.
Following Mauchly's death, Kay carried on the legacy of the ENIAC pioneers by authoring articles, giving talks (frequently along with Jean Bartik, with whom she remained lifelong friends), and making herself available for interviews with reporters and researchers. She was inducted into the Women in Technology International Hall of Fame in 1997 along with the other original ENIAC programmers, and she accepted the induction of John Mauchly into the National Inventors Hall of Fame in Akron, Ohio in 2002.
Antonelli was diagnosed with inoperable cancer in early 2006, and died in April at the age of 85.
- While her official date of birth is always listed as February 12, Antonelli herself suspected she may have been born on February 13, the date having been "fudged" by her family according to a common practice out of Irish triskaidekaphobic superstition.
- Autumn Stanley: Mothers and Daughters of Invention: Notes for a Revised History of Technology, The Scarecrow Press, 1993, pp.442/443, ISBN 0-8135-2197-1
- Bernadette Schell, Clemens Martin: Webster's New World Hacker Dictionary, Wiley Publishing Inc., Indianapolis/Indiana, 2006, p. 16
- Thomas J. Misa: Gender Codes: Why Women Are Leaving Computing, IEEE Computer Society, published by John Wiley & Sons Inc., Hoboken/New Jersey, 2010, p. 121, ISBN 978-0470-59719-4
-  IEEE Annals of the History of Computing: The Women of ENIAC
- Martin Gay: Recent Advances and Issues in Computers, The Oryx Press, Phoenix/Arizona, 2000, pp.106/107
- J.J. O'Connor/E.F. Robertson: Kathleen Rita McNulty Mauchly Antonelli, 
- WITI Hall of Fame
- Biography from the University of St Andrews, Scotland
- Oral history interview with Frances E. Holberton – Holberton was, with Antonelli, one of the six original ENIAC programmers. Charles Babbage Institute, University of Minnesota.
- Death of Donegal's Computing Pioneer