She runs across the street and picks up the crying boy, who was making his way around a corner, bringing him on board her bus.
As the bus sat waiting for police and EMS responders to arrive, one passenger walked up to Ivic as she held the baby in the driver's seat.
She suddenly stopped the bus and dodged traffic to rescue the baby.
Video shared by the Milwaukee County Transit System shows bus driver Irena Ivic stopping her bus to rescue a crying child that was wandering barefoot down a highway overpass early on December 22, 2018. Besides being cold and scared, the baby was unharmed.
Police said the girl had been left outside by her mother, who is thought to be suffering from a mental health crisis.
ABC7 reported the boy was later reunited with his father.
A look of concern covers Ivic's face in the video when she first sees the infant meandering along the sidewalk of the highway overpass near 4th and Mitchell streets.
"She did a lot more than her job. I don't take that kind of compassion for granted", said County Executive Abele.
"I have two lovely children and I am a human being and I find the child upset, and if I had the same situation again, I will, of course, again get help".
Originally from Serbia, Ivic has lived in Milwaukee for over a decade and been an employee of MCTS for the past five.
This is the ninth time a lost or missing child has been found by a Milwaukee County Transit System driver in recent years, according to the release. This post has been updated.