Born in New York City, New York, Loomis was the son of Henry Patterson Loomis and Julia Stimson.
In 1917, with the United States' entry into World War I, he voluteered for military service. He was commissioned as a captain and rose to the rank of lieutenant colonel He worked in ballistics and invented the "Aberdeen Chronograph".
In the 1920s, Loomis collaborated with his bother-in-law, Landon K. Thorne, to take Bonbright and Company from the verge of bankruptcy to a preeminent U.S. investment banking-house, specializing in public utilities. In the process, Loomis became very wealthy.
During this period, Loomis also established Tuxedo Park Loomis Laboratory. The facility was visited by prominent scientists, including Albert Einstein, Niels Bohr, Werner Heisenberg, Vannevar Bush and James Franck. In its early years, the laboratory's work focused on timekeeping.
Just prior to the 1929 stock market crash, Loomis liquidated his holdings, thereby avoiding financial ruin. He used his personal wealth to support scientific research over the ensuing two decades.
Loomis chaired the Microwave Committee of National Defense Research Committee and invented LORAN. He also made a significant contribution to the development of Ground Controlled Approach technology (a precursor of today's Instrument Landing Systems), which used radar to permit ground controllers to "talk-down" airplane pilots when poor visibility made visual landings difficult or impossible.
He was elected to the National Academy of Sciences in 1940.
Loomis received honorary degrees from Wesleyan University (D.Sc 1932), Yale University (M.Sc 1933), and the University of California (LL.D 1941).