Co-creator of the ILLIAC series, culminating with the ILLIAC IV supercomputer, the largest and fastest in the world, Meagher completed his physics degree at the University of Illinois in 1949. He had previously worked at the MIT lab on the development of naval radar. He promptly joined the physics faculty and was appointed chief engineer. Working with electrical engineer Louis N. Ridenour and mathematician Abraham H. Taub, they created the ORDVAC, which was the first of two computers built under contract at the University of Illinois.
ORDVAC was completed in the spring of 1951, and in the fall it was delivered to the US Army's Aberdeen Proving Grounds. As part of the contract, funds were provided to the University of Illinois to build a second identical computer known as ILLIAC. In 1952 Meagher and his colleagues developed ILLIAC I, the first digital computer built and owned entirely by an educational institution. It weighed five tons and contained 2,800 vacuum tubes.
The ILLIAC series continued with ILLIAC II, a transistorized computer, and culminated in the mid-1960s with the ILLIAC IV supercomputer, the largest and fastest in the world.
