Originally from Endicott, New York, Appel attended the Art Institute of Pittsburgh, majoring in computer animation.
After moving to New York City, Appel started taking classes at the Upright Citizens Brigade Theater, "and it wasn’t long before I was basically living at the theatre.
He subsequently wrote for Crank Yankers and Human Giant, then went to work for the comedy website Funny or Die.
"[3] Appel would go on to extensively direct series television, with credits including Eagleheart, NTSF:SD:SUV, The Office, New Girl, Selfie, Brooklyn Nine-Nine, Son of Zorn, Die Hart and The Afterparty.
[4] In 2022, Appel made his feature directorial debut with Weird: The Al Yankovic Story, based on a Funny or Die short that he directed twelve years earlier.