Arthur Spiderwick (born in 1890) is Lucinda's father. He wrote a book titled Arthur Spiderwick's Field Guide to the Fantastical World Around You. His daughter, Lucinda, described it as "his life's work". The book documents biographies and characteristics of magical creatures as well as their vulnerabilities, weaknesses, and flaws.
In the film, a large group of Sylphs capture him and forcibly transport him to a mysterious glade where they alter his perception of time. However, in the novels, the Green-eyed Fairy restricts him to the unseen realm, where he spends decades but stays the same age. When Jared, Simon, and Mallory came to the unseen realm, Jared asked Arthur for advice on how to defeat Mulgarath, but Jared didn't find his advice that helpful. At the end of The Wrath of Mulgarath, Jared and the others go to the Unseen realm, where Arthur sees Lucinda for the first time in years. Lucinda is a frail old woman, while he has stayed the same age he was when he left. Faerie magic had kept him alive for too long and he stepped foot on the real world and turned to dust. But in the movie, Lucinda is reunited with her father and they become flowers and are blown by wind into the sky, which represents them passing on to the afterlife.