In 1960, he became the Wisconsin state commissioner of taxation, and he supported John F. Kennedy for President.

As Postmaster General, Gronouski promoted the original five-digit zip code system, and worked to end racial discrimination against postal employees.

[3] After he left the Cabinet on November 2, 1965, President Lyndon B. Johnson appointed him to be Ambassador to Poland.

After President Richard M. Nixon took office in 1969, Gronouski became founding dean of the Lyndon B. Johnson School of Public Affairs at the University of Texas at Austin.

He is interred in Allouez Catholic Cemetery and Chapel Mausoleum in Green Bay, Wisconsin.