He's also been a Mercedes simulator development driver since 2011 - the absolute ultimate in motorsport video "games" - so the test was a great opportunity for him to turn a lot of hard work into reality.
"It [a Formula One car] was quicker than I remember it but I felt a lot more prepared than when I was only 18," said Hartley. "The quickest car I had ever driven back then was a Formula Three and I went straight from that into an F1 car.
"Some things don't change. The forces that you go through are still the same. I felt very comfortable and feel like I did a really good job. It was a very special feeling to be back behind the wheel, that's for sure.
"I'm happy with my performance and pleased that it only took a couple of runs before I was on the pace as it's been three years since I last drove a Formula One car. I'm very grateful to Mercedes AMG Petronas for the experience and really enjoyed working with the team."
The three-day test gave Mercedes the chance to try out a series of new parts, including a revised exhaust system, and team principal Ross Brawn was pleased with how it all went.
"Today sees the conclusion of a successful three-day test for the team in Magny-Cours with our young drivers Sam Bird and Brendon Hartley," said Brawn, after the final session. "This was an important test, allowing both drivers some important track time in a Formula One car, and enabling us to continue our development programme with the evaluation of our new upgrades.
"Both Sam and Brendon have driven very well, providing good and consistent feedback, and I would like to thank them for their efforts and diligence over the last three days. We now have a wealth of data from the test to study back at the factory over the next few days and in advance of the forthcoming race in Singapore."
Simulator testing plays a vital role in most forms of motorsport these days, especially in Formula One where actual on-track testing time is strictly limited. It would be fair to say every part of an F1 car is rigorously tested on the simulator before it makes its way on to the circuit. However, it doesn't matter how close a simulator can get to be a race car it will never be the real thing.
Hartley would have experienced an array of sensations as he lowered himself into the cockpit, strapped in, hit the start button and rolled out of pit lane to actually feel the car working on tarmac.
"When braking and accelerating you're pulling more than five Gs," he said. "By the end of the day you're starting to feel your neck even under braking. When you're coming up to a corner and changing down from seventh gear to first you're decelerating from about 320km/h to almost nothing in about 90 metres.
"In the first few laps your brain doesn't really understand how you can brake so hard and then get through the corner so fast. It doesn't take too long, though, for the brain to adjust and away you go.
"The back takes a beating because you're cornering at such high speed with high G-forces at work. The forces are trying to pull you out of the seat and you have to use all your core muscles to stay in there.
"Even though you're strapped in quite tight, you still have to hold yourself in there. It's difficult to explain, as people can't see how many forces are going through your body at such high speed with so much grip.
"There's still a big difference [between simulator and car] and obviously reality is one of them. In the simulator when you crash nothing happens so you lose that fear, and there's nowhere near as much force on your body. The simulators are getting closer all the time though."
Every part that goes on to the car is exhaustively tested via the simulator first. Hartley is testing and passing on opinion about everything to do with the car. All the data gathered go to making the car better.
Not only did Hartley earn his drive in a car he's helped to develop, he's one of the few involved at the top end of motorsport who has played in an F1 car.
"Other than the drivers there's not many people in motorsport who get a chance to drive an F1 race car," he said. "Per year each team only gets three days to test young drivers who haven't competed in a race. That's not a lot and I've done four official tests now for three different teams, which is pretty special."
He's worked damn hard over the last couple of years behind the scenes and not a lot of people realise the contribution he has made and the faith the Mercedes team have in him.
"I'm not going to speculate what might happen from now on, but I've still got my foot in the Formula One door."
