[2] Willingham got his start from the late 1970s to early 1980s as a staff artist for TSR, Inc., where he illustrated a number of their role-playing game products.

He was an interior artist on White Plume Mountain, Slave Pits of the Undercity, Ghost Tower of Inverness, Secret of the Slavers Stockade, Secret of Bone Hill, Palace of the Silver Princess, Isle of Dread, The Mansion of Mad Professor Ludlow,[3] Food Fight,[4] In the Dungeons of the Slave Lords, the original Fiend Folio, Descent into the Depths of the Earth, Assault on the Aerie of the Slave Lords, Against the Giants, Queen of the Spiders, Realms of Horror, and the second and third editions of the Top Secret role-playing game.

[5] He contributed stories to Green Lantern and started his own independent, black-and-white comic book series Coventry which lasted only 3 issues.

[11] At the 2007 Comic Con International, he announced that he would be writing Salvation Run, a mini-series about supervillains who are banished to an inhospitable prison planet.

[19] In late 2010 (with cover dates January–April 2011), Willingham wrote the four-issue mini-series Warriors Three for Marvel Comics, illustrated by Neil Edwards.

[20] At 2013 New York Comic Con it was announced that Willingham would be writing a seven-part mini series for Dynamite Entertainment (with art by Sergio Fernandez Davila).