As a screenwriter, he is known for writing the 2018 thriller Unsane directed by Steven Soderbergh and starring Claire Foy and Juno Temple, as well as the family comedies Max Keeble's Big Move, Just My Luck and The Spy Next Door.

[11] and Bad Eminence (And Other Stories, 2022), which Andrew Gallix in The Irish Times called "this year's cult classic," writing that the novel was "hilarious, exhilarating and mind-blowing".

Publishers Weekly said of the book "Halfway between the mind of God and a vivid dream, Everything Flows is proof that there remain new places to go, both on paper and in the known universe.

Greer has written or co-written several movies, including Max Keeble's Big Move[14] (Disney, 2001), Just My Luck[15] (20th Century Fox, 2006) starring Lindsay Lohan, the Jackie Chan vehicle The Spy Next Door[16] (Lions Gate, 2010) and Unsane,[17] directed by Steven Soderbergh, and starring Claire Foy and Juno Temple (Fingerprint Releasing, Bleecker Street, Regency, 2018).

In February 2018, Deadline Hollywood reported that Greer was set to write a script called Planet Kill for Soderbergh to produce.