Shocked onlookers swam across the boating lake at Stanborough Park, Welwyn Garden City, on Sunday after the car became submerged in the water.
The pair were put in a boat and taken to land. The man, who is from Potters Bar, was taken to hospital but discharged later in the day.
Police said he was "shaken" but "not injured".
As reported in the Welwyn Hatfield Times , video from Alex Kerr on YouTube shows three people in the lake, trying to smash a window of the Nissan Qashqai.
A spokeswoman for Hertfordshire Police said they eventually helped the man and his dog out of the car via other means.
She said the force was called at about 12:45 BST and the car was pulled from the water by the fire service shortly after 16:00.
