Eventually, Tomasi would transition into writing, most of his work being with characters he'd been editor for, his first project being Black Adam: The Dark Age, fitting given his tenure as JSA editor. The series was drawn by Doug Mahnke, who would be a frequent collaborator of Tomasi's in the future. He would go on to write most of Green Lantern Corps, from the Sinestro Corps War story to Blackest Night, featuring Guy Gardner and Kyle Rayner. His GL work would establish his penchant for sentimental, humanising stories in the superhero genre. His run was largely drawn by Patrick Gleason, his most frequent collaborator from then on. After Blackest Night, Tomasi co-wrote Brightest Day with Geoff Johns along with the Emerald Warriors GL spin-off, which featured Guy Gardner and a ragtag band of Green Lanterns. Tomasi would then write Green Lantern Corps for another 21 issues in the New 52, this time featuring Guy Gardner and John Stewart.
These days, Tomasi is primarily known for his very well-received Batman works. Tomasi wrote the last two arcs of Nightwing before its cancellation, as Dick Grayson would go on to become Batman. He also wrote the last arc of Batman and Robin, launched by Grant Morrison as the middle act of his ''Batman'' saga before the New 52, which featured Dick Grayson as Batman and Damian Wayne as Robin. Tomasi also wrote the Batman tie-in to Blackest Night, which featured the two going up against a resurrected clone of Bruce Wayne. The New 52 Batman and Robin series, featuring Bruce Wayne and Damian Wayne, was written by Tomasi with art by Gleason, and is a large part of why Damian Wayne was Rescued from the Scrappy Heap for many readers. The run focused largely on Bruce and Damian learning to be a parent and child, respectively. After 43 issues, the series ended, with Patrick Gleason going on to write Robin: Son of Batman, a sequel series. Tomasi would later do some small Batman work here and there: a brief Detective Comics run after Brian Buccelato quit the title, and the Darkseid War tie-in for Batman.
After Batman and Robin, Tomasi moved to the Superman books, his first major project not involving a character he was editor for. He wrote some Superman/Wonder Woman, but is more well-known for writing the entire The Final Days of Superman crossover, which featured the death of the New 52 Superman. Tomasi would go on to write Superman (Rebirth) (co-written by Gleason), the DC Rebirth relaunch of the title that features the pre-Flashpoint version of Clark Kent and Lois Lane, along with their son, Jon Kent. The art team consists of Patrick Gleason and Doug Mahnke, with Jorge Jimenez doing some breather episodes. The run has been incredibly well-received, and also has a spin-off, also written by Tomasi — Super Sons, drawn by Jimenez. This spin-off, fittingly enough, features Jon Kent's adventures alongside Damian Wayne. Tomasi left the title so DC could make room for Brian Michael Bendis' inaugural work for the company, and his Superman run was cut short with some unresolved plot threads. Not soon after, Tomasi returned to the Batman books to write Detective Comics, writing the arc leading up to the 1000th issue of the title, and will continue to write the title afterwards.
Comics written by him:
- JSA Classified
- Green Lantern
- Wonder Woman: Black and Gold: "Hellazapoppin" (writer)
Tomasi's work includes examples of:
- Darker and Edgier: His works tend to be more violent than most, albeit always to serve a purpose. Black Adam: The Dark Age featured zombies and gruesome decomposing corpses, Batman and Robin had tons of violence, even for a Bat-book, and Superman has some dark sci-fi imagery. Even Super Sons has an entire family murdered in the second issue even if it was revealed that they were simply hyper-realistic robot duplicates designed to terrify the originals while they watched.
- Lighter and Softer: On the other hand, his stories tend to feature lots of emotional moments and have relatively small stakes compared to other comics in their line. Super Sons, however, is both lighthearted in its themes, story and art.
- Parents as People: It started with Batman and Robin and continued into his Superman work. Batman and Robin has Bruce Wayne learning to be a parent while raising his son, while much of Superman's focus is on Clark raising his son and dealing with problems he can't punch away. Tomasi himself has said that his own relationship with his son has a big impact on his writing.