But inside, the 43-year-old mother of four was anything but happy or calm.
"I just reassured them that we were okay, we were getting out and we were going home and we were going to be fine," she said.
Cahill had just crossed the finish line and was grabbing a water bottle when the bombs exploded.
"I turned around to see what was happening and when I saw the clouds of smoke and people starting to rush from the finish line up the street, I started to take off down the other way," she said.
Though Cahill had felt exhausted before reaching the finish line, a sudden surge of adrenalin gave her the energy to run through the panicked crowd and find her family at their planned meeting place.
Through text messages, she learned that her father, brother and sister, who'd been at Mile 21 when she crossed, were also safe.
And they were able to text her two older children, who were waiting back home in Buckingham Township.
Kelly was certainly not the only runner affected by the events in Boston.
Eric Bofinger, who works as a trainer at BucksFit in Richboro, Pa. said he was already on the subway. But he will never forget what happened.
His jubilation at beating his own marathon personal best by a minute quickly gave way to many other emotions.
"We wanted to rejoice in our triumphs and everything that we did," he said. "But on the other hand we were really sad and upset that something that's so precious to us as the Boston Marathon could be tarnished like this."
Both Eric and Kelly said that before Monday, they'd never even considered the issue of safety at the finish line.
Both say it's too early to know if they'll compete in Boston again.