The actor has been diagnosed with angioimmunoblastic T-cell lymphoma, and began receiving chemotherapy to try and tackle the disease.
Just three months after starting the treatment, the chemo stopped working on Neill's cancer, prompting doctors to try and treat him with a rare anti-cancer drug.
Neill now requires infusions every two weeks, but amazingly the drug seems to be working for him, and he's been in remission for 12 months.
The days after his treatments are 'very grim and depressing', he admitted to Australian Story, as they leave him feeling like he's gone 10 rounds with a boxer.
However, while Neill knows the treatments are 'keeping [him] alive', he's determined not to spend too much time thinking about the disease.
"I know I've got it, but I'm not really interested in it," he said. "It's out of my control. If you can't control it, don't get into it."
Neill recognizes now he has 10 days after the effects of his treatment have worn off to enjoy his time before the next infusion.
"Ten days," he said, "in which I could not feel more alive or pleased to be breathing and looking at a blue sky".
Neill has been told by doctors that the drug will stop working at some point, and he's 'prepared for that'.
He admitted death would be 'annoying', because he's got more to do in life, but after thinking deeply about mortality following his diagnosis, he's now 'not remotely afraid' of death.
Retirement, on the other hand? Well, Neill said that 'fills [him] with horror'.
The actor could not continue acting while receiving chemotherapy, which is why he decided to start writing his memoir.
"I started to think I better write some of this down because I'm not sure how long I have to live," he recalled. "I was running against the clock."
Now in remission and determined to keep working, Neill began filming for the adaptation of Liane Moriarty's novel, Apples Never Fall, before production was shut down as a result of the actors' and writers' strike in the US.
Neill has also been filming for season two of the mini-series The Twelve.