Robert Kirkman's first ever story was called Battle Pope. Battle Pope is a parody of superhero comics following the main character "Jesus H. Christ", the pope.
This comic ran from 2000-2007, and got an animated depiction on Spike TV. The years of 2001-2004 was when Kirkman made his most successful comics, Invincible and The Walking Dead .
Since then The Walking Dead has had 32 volumes, 3 compendiums, and an AMC live action depiction. With Invincible having 25 volumes, 3 compendiums, and an Amazon prime exclusive animated show.
Marvel Zombies, part 1 and 2, went from 2006 to 2008. It tells the story of what would happen if a zombie virus, like The Walking Dead, had ravaged the Marvel comics universe. Marvel Zombies now has a animated depiction in the series called "What if..." on Disney Plus.
Outcast, a horror comic series made by Robert Kirkman and Paul Azaceta, went on from 2014 to April of 2021. It has a television adaptation that was unfortunately cancelled by Cinemax in 2018.
Kirkman has plenty of ongoing work. Such as, Oblivion Song, which is a supernatural science fiction that takes place in an apocalyptic Philadelphia, where the main character is trying to rescue the few amounts of remaining survivors.
Oblivion Song was first published by Image Comics in March of 2018. In February of 2021 it was announced that Issue #36 will be the end of the story, it is current on it's 24th issue.
DIE!DIE!DIE! is an ongoing comic made by Kirkman, Chris Burnham, and Scott M. Gimple. The story behind it is a group of assassins trained by the U.S government, to influence the world through assassinations.
The first issue of DIE!DIE!DIE! was released on July 11th, 2018. The most recent issue was released on March 24th, 2021. There's no telling when the story will end.