Starting his career first as a manga artist"s assistant (self-described as "horrible" and "lazy" in said assistant position), and later an H manga artist, he went on to enjoy somewhat limited success with other relatively unknown manga titles such as Angel Dust, Coyote, Gun Mania and Hi-Tension. His first major success came with his manga series Hellsing, which got its start and was subsequently serialized in a monthly manga magazine, Young King OURs, towards the latter half of 1997. However, Hellsing was not the earliest Hirano series to be published in Young King OURs monthly.
In 1996, the same year Hellsing"s precursor, The Legends of Vampire Hunter, was first released as a single H short story in Heavenly Pleasure (a monthly H-centric manga magazine), another World World War II-based short story named Hi-And-Low was being published in Young King OURs by a then lesser-known Kouta Hirano.
The story takes place primarily at a train station in Russia and features two female characters that are strikingly similar to Integra Helsing from Hellsing and Yumiko/Yumie in Crossfire. And who are, in actuality, undercover Axis spies in-league with one another for a common purpose: the success of Operation Barbarossa.
This story saw ink in one issue of OURs before being discontinued in favor of Hellsing itself. A number of Hirano"s older works are now considered collector"s items due to the small number of them that exist.
Many characters from Hellsing appear in his previous works and, as mentioned above, there is a rare hentai prototype of Hellsing titled The Legends of (the) Vampire Hunter.
At Otakon 2006, he said in an interview that in about a year and a half to two years, he will finish Hellsing and move on to a different project which he says will be kept a secret until the time comes. This statement was proven true when Hellsing ended with 95 chapters in October 2008. Hirano has since begun a new series, Drifters, which was published in April 30"s issue of YKO and which Hirona keeps working on to this day.
Hirano contributed a piece of fan art to "Project G", a dōjinshi tribute to the manga, Genshiken.
The piece features the character Madarame resting and looking at his cell phone. Hirano has also been part of a doujinshi circle titled GUY-YA, consisting of himself and Read or Die manga artist Shutaro Yamada.