Greaney has degrees in political science and international relations, subjects which became major focuses of his writing career.

After finishing one of them, titled Goon Squad[4] and was primarily about the aftermath of the Bosnian civil war, he gave the first 20 pages of his work to his favorite author Ralph Peters's agent, Scott Miller, in a book conference in September 2006.

But he urged Greaney to write another one based from a character in Goon Squad named Court Gentry,[4] which would later be The Gray Man.

[2][5] The national success of The Gray Man made Greaney quit his job in the medical devices company and make the transition to becoming a full-time writer.

The film adaptation of The Gray Man was directed by Joe and Anthony Russo for Netflix, with Ryan Gosling as Gentry and Chris Evans as nemesis Lloyd.

[13]Greaney's first standalone work, a military thriller titled Red Metal and co-authored with Lieutenant Colonel H. Ripley Rawlings IV, USMC, was released on 16 July 2019.

[14] In 2020, Greaney and Rawlings both announced via social media and web interviews that they had been contracted by Penguin Publishers to write the next in the Red Metal series.

Greaney's audiobook thriller Armored was released on 9 December 2021, and was adapted by Sony Pictures and producer Michael Bay.