Starting as an editor at DC in 1993, and working on such titles as Green Lantern, the Batman titles, Aquaman, Hawkman, and JSA, he was an occasional writer on various titles, including JSA, The Outsiders, Steel and The Light Brigade. In 2003, DC promoted him to Senior Editor.

In 2007, Tomasi left his fifteen-year role as an editor and transitioned to one of writer. He began on the limited series The Dark Age, and he is currently the regular writer on the monthly Emerald Warriors and Outsiders titles, and wrote the Nightwing title for 14 issues until its cancellation due to the Battle for the Cowl event. He has also written the Final Crisis tie-in "Requiem", a tribute to the Martian Manhunter.

Recent projects including writing, with Keith Champagne, a new DC Comics creator-owned title The Mighty, as well as the Green Lantern Corps through The Blackest Night summer event title. As of 2010, Tomasi co-writes the Blackest Night follow-up title, Brightest Day, with Geoff Johns.

Comics writing work includes:

• Steel #28, 33 (with Andrew Robinson and Jim Aparo, DC Comics, July, December 1996)
• The Light Brigade (with art by Peter Snejbjerg, 4-issue mini-series, 2004, tpb, 200 pages, January 2006, ISBN 1401207952)
• The Outsiders (vol. 3) #26-27, with Will Conrad, DC Comics, September—October, 2005)
• Classified #8-9 (with pencils by Don Kramer and inks by Keith Champagne, DC Comics, 2006, tpb, Honor Among Thieves, 128 pages, January 2007, ISBN 1401212182)
• The Dark Age (with art by Doug Mahnke and Christian Alamy, 6-issue mini-series, DC Comics, 2007, tpb, 144 pages, June 2008, ISBN 1401217869)
• Green Lantern Corps #18-20, 23-47 (with art by Patrick Gleason, DC Comics, January 2008-May 2010)
• Nightwing #140-153 (with pencils by Rags Morales and Don Kramer and inks by Michael Bair, DC Comics, March 2008-April 2009, tpb, Freefall, 168 pages, 168 pages, ISBN 1401219659)
• "Final Crisis: Requiem" (with art by Doug Mahnke and Christian Alamy, one-shot, DC Comics, 2008)
• The Mighty (with co-author Keith Champagne and art by Peter Snejbjerg, 12-issue limited series, DC Comics, forthcoming)
• The Outsiders (vol. 4) #15- (with art by Andy Kubert, DC Comics, 2009, forthcoming)
• Blackest Night: Batman (with art by Adrian Saef & John Dell, 3 issue mini-series, DC Comics, 2009)