Lacy, a specially trained explosive detection canine, was the first-ever canine to be certified to screen passengers for the Transportation Security Administration (TSA). She graduated from TSA's inaugural class after completing an intensive training course at Auburn University in 2011.
On Tuesday, Lacy completed her last shift at Midway and was presented with a certificate of appreciation.
"She's the best partner you can ask for. She's been by my side the whole time, and I enjoy working with her every day," said Diana Kremer, Lacy's trainer.
Kremer is adopting Lacy so she can enjoy her retirement and by staying home full-time with another dog and cat.
Lacy spent her entire career working at O'Hare and Midway and it is estimated that she screened more than 1 million passengers during that time, according to officials.