Fifty-five-year-old Tim said he parked at his partner's house in Hayley Road in Lancing and took her dog for a short, 15-minute walk.
When he returned to his car, which he admitted he had left unlocked, he found a hooded teenager, wearing gloves, sat in the driver's seat.
"I was stunned," he said.
"The lad said 'your name's Tim, isn't it?' and, when I asked him what he was doing in my car, he said he was waiting for someone.
"He was obviously making up a story but he wasn't overly panicked.
"I thought I am going to grab him, but I'm not a man of violence and I had my dog with me. You never know what the outcome of that could be. It's just shocking really.
"I'm a bit shaken up when I think about what may have happened but I am glad no-one was hurt, including him. He's just a young boy and it's sad he felt he had to do this sort of thing to make a bit of money."
Tim tried to take a picture of the boy, who he said was around 16 to 18 years old, but he covered his face before he got a chance.
Despite the jarring experience, he said the intruder was actually surprisingly polite, did not swear and calmly left the scene once confronted.
He has urged people to always lock their cars and said he would be in the future, as it was likely the teenager had been checking people's doors.
The police were also praised for their swift response, arriving within 15 minutes, Tim said, and willingness to investigate.