García Seijas was born in Ramos Mejía, and at age 17 he made his debut in comics by providing art for the series Bill y Boss, published in the Argentine magazine Totem.
García Seijas later collaborated substantially for Columba, a publisher specializing in romantic comics books for women and in film adaptations; these include Helena, created by Robin Wood, which was turned into a TV series in the 1980s.
García Seijas also worked for Columba's main rival, Record, with adventure/mystery/western works such as Skorpio and Many Riley (written by Ray Collins), El Hombre de Richmond ("The Man of Richmond", script by Andrea Mantelli), La estirpe de Josh (written by Alfredo Grassi) and Los aventureros (script by Carlos Albiac).
In 1990s García Seijas started to work directly for Eura Editoriale and then for Sergio Bonelli Editore, Italy's largest comics publisher.
In 2011 Bonelli published his Tex large-format one-shot Le iene di Lamont (2011), with script by Claudio Nizzi.