His works spanned a range of media including watercolors, oils and acrylics, lithographs, and sculptures.
His subjects included landscapes, horse and sailboat races, urban and industrial scenes, burlesque and ballet dancers, nudes, still life, and narrative works.
[1] During the politically turbulent 1960s Pen was employed as a court artist to document trials because cameras were not permitted in the courtroom.
[5] Pen's professional affiliations included The American Watercolor Society, National Academy of Design, and the Union League Club of Chicago.
A 2003 exhibition catalogue description described his style: "Rudolph Pen understood on an instinctive and molecular level that all things move.