Garrett was murdered in the episode "This Is A Dark Ride". He is the second person that saw part of what happened that night to be killed.

Who killed Garrett PLL?

He is shot to death by Darren Wilden, who was working alone and dressed up as Queen of Hearts because he had discovered that the latter was a dirty cop and was going to tell. His body is locked inside a wooden crate, along with Aria.

Why did Garrett get killed on PLL?

9 Least Believable: Garrett Reynolds

Unfortunately, this is the last time the viewers see him alive as the next scene he features in shows that he has been murdered dead. It turned out that he was shot by Detective Darren Wilden, who killed him after he discovered Garrett was about to expose his corrupt behavior.