He was one of the founders of the Delaware County Institute of Science and published several books describing 194 new species of birds.

[2] In 1833, Cassin, along with 4 colleagues, founded the Delaware County Institute of Science in Media, Pennsylvania.

[4] From 1846 to 1850, Thomas Bellerby Wilson, a wealthy patron of the academy, became interested in the department of ornithology and procured a collection of over 25,000 birds.

[8] This exceptional collection of birds allowed Cassin to become the leading ornithological taxonomist in the world.

[10] He also co-authored Birds of North America (1860) with Spencer Fullerton Baird and George Newbold Lawrence.

[13] His collection of 4,300 birds was purchased for $500, equivalent to $11,445 in 2023 dollars, by John Whipple Potter Jenks for Brown University's museum of natural history.