The Kiwi, who made his F1 debut at the 2017 US Grand Prix, spent most of his first full season facing scrutiny over his future in the sport, as he struggled to match teammate Pierre Gasly on the drivers' standings, where he finished 19th out of 20 drivers.
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The 29-year-old was cut by Toro Rosso and replaced by promising British-born Thai Alexander Albon within 24 hours of the Abu Dhabi season finale in November.
Toro Rosso also opted to bring back former driver Daniil Kvyat, with Gasly earning promotion to the Red Bull team for 2019.
Writing for The Players Tribune, Hartley said discovering the truth to this speculation during round six of the season was one of the toughest moments in his motorsport career.
"What I will remember most about it [the Monaco Grand Prix weekend] is walking down to the paddock to meet with the media on the Wednesday before the weekend started and receiving a bunch of questions about my future," he explained.
"Here I am, a handful of races into my F1 career, and I'm being asked about the end.
"The worst part of that day, though, was finding out there was some truth to the rumours. After a few races, there were some people, it appeared, who didn't want me there.
"I'll be honest, this was a bit of a shock.
"After entering F1 with a wealth of experience, two World Endurance championships, a win at Le Mans and out-qualifying my teammate two out of the first three races, it was hard to for me to believe that there was talk of my being replaced so early.
"I walked back to our apartment that night, looking at the walls of the Monte Carlo circuit, knowing that, if I binned it, if I made contact with those walls this weekend, my F1 career might end in a few days.
"I knew every practice session carried more weight for me. Every lap time, every result was going to be under scrutiny and could be used against me to leverage my seat.
"That's a unique type of pressure that I hadn't quite experienced before."
Hartley's season was riddled with bad luck, with some frightening crashes and engine failures, but there were some shining moments, including a sixth-place finish in qualifying in Japan and a ninth-placed finish in the US Grand Prix.
But Hartley said the "looking-over-my-shoulder feeling" stayed with him all year and he found himself thinking of legendary Kiwi driver Chris Amon, a driver well known for his misfortunes during an F1 season that included "hitting birds, being taken out on the first laps, engine penalties, suspension failures and other issues that weren't always mentioned in public".
Going into the final race of the season in Abu Dhabi , Hartley had no idea about his future, but he found out very quickly after finishing 12th at the Yas Marina Circuit.
"Like the fans, I had no idea what was going to happen," he added. "That's the thing about the politics in F1 - it can be a little bit... awkward.
"Everyone sort of walks on eggshells and there isn't always clarity. So I just did all I could - my job.
"I out-qualified my teammate and drove to 12th on Sunday night.
"An hour later, I was summoned to a meeting and a few minutes after that, I was no longer an F1 driver.
"I walked down to the garage and I told some of the guys that I wouldn't be coming back. That was tough.
"I was a proud member of Toro Rosso and Honda, and saying goodbye that day was one of the hardest things I've had to do."