The New Zealander came in for the final few races of the 2017 season, and continued throughout the 2018 season at the junior Red Bull squad.
Hartley, on the other hand, is out. He has been replaced by Thai F2 driver Alexander Albon.
He still has goals of what he wants to achieve in the sport, but it could be too late for that.
“Obviously I feel like I have unfinished business in Formula One,” Hartley said.
“I’ve finished the season much stronger than I started.”
Hartley also believes that it was not all through his own doing, and that bad luck played a part at different points of the season.
“I had some tough times, a lot of pressure put on,” he continued.
“I think I reacted well and I’ve come out the end of the season a much stronger individual, driver, and really happy with my progression through the season.
“I haven’t always had opportunities to score points, sometimes outside of my control, definitely some missed opportunities in the beginning.
“But I’ve been a good team player all year and I know I’ve very much contributed to the performance.”