While in the US, he published his first article, "New England Terms for Poached Eggs," which received media coverage by the Associated Press among others.
In 1944 he became a naturalized US citizen and from 1943 to 1945 he served in the United States Army, working on the development of military dictionaries.
In 1963, he received an offer from the Linguistics Department at the University of California, Berkeley, where he spent the remainder of his academic career.
Penzl's research included a wide variety of topics, but his main interests were Germanic historical phonology.
He wrote over 250 research articles and published 11 books, many of which have become standard works for students of Germanic Philology.