Some of Declan's earlier work includes, but is not limited to, the first 16 issues of the 28 Days Later ongoing and various Planet of the Apes comics for Boom! Studios. He was also the artist for Parts 4-6 of the "Icelandic Trilogy" story arc in Northlanders over at Vertigo. He also illustrated graphic novel adaptations of Sweeney Todd and Frankenstein for Classical Comics.

He began drawing for Marvel with Thunderbolts where he drew the interior art for the Shadowland tie-in issues. This eventually led to him drawing the interiors for the title full-time starting with the Fear Itself issues, as well as having drawn a spin-off title: Crossbones #1. The art chores for Thunderbolts alternated between Declan and Kev Walker. Declan continued to draw the book when it is rebranded as Dark Avengers following #175 but left the book shortly afterwards, his final issue was #178.

Declan pencilled Rick Remender's final issue of Venom and will continue to pencil alongside new writer Cullen Bunn.