He collaborated with writers Nancy White Carlstrom, on the Jesse Bear books, and Jane Yolen, on the Commander Toad series.

[3][4][1] He then attended Pratt Institute in Brooklyn where he obtained a Masters of Fine Arts degree in printmaking and painting.

[1] Degen's working life has included designing advertisements, teaching art, making prints, painting scenery for opera productions, and directing a lithography studio in Israel.

[5][6] From the 1960s to the late 1980s, Degen taught art in public schools of New York City and worked as a freelance artist and illustrator on the side.

[4][5] He illustrated Jane Yolen's Commander Toad series, beginning with Commander Toad and the Planet of the Grapes in 1982 ("an early success" of his career), and Nancy White Carlstrom's Jesse Bear series, beginning with Jesse Bear, What Will You Wear?

[4][5] In the mid-1980s, he became the illustrator for The Magic School Bus book series written by Joanna Cole.