Even as his strength ebbed, he mapped out scenarios for the leadership convention that might follow and for the challenges his successor would inevitably face.
"What he wanted from us was advice, plans on what would happen if he wasn't able to be back in the fall ... and what would happen if he were to pass away," said Anne McGrath, Layton's chief of staff.
It had been less than a month since a frail and hoarse NDP leader told a news conference he was stepping aside temporarily to fight a new cancer. He had already spent months fighting off prostate cancer, enduring the grind of the election trail.
Layton did not waste time after addressing a saddened and stunned nation, McGrath recalls.
"He said, 'OK, now that that's done, I'm going to pull my medical team together this week and challenge them to come up with a plan,'" she said Monday in an interview.
"So he knew it was tough, he knew it was an uphill battle, and it certainly was an uphill battle over the summer."
McGrath, who had worked closely with Layton since his successful 2002 leadership run, spoke to him daily. One of those calls came when he was receiving treatment at Toronto's Princess Margaret Hospital.
"He started to talk and then he said, 'Oh, I have to go, the medical team is here now,'" she remembered. "Then I was back on the phone with him probably 15 minutes later."
There were good days and bad days. Through it all, the unwaveringly positive politician's spirits did not flag.
"The Jack Layton that people see in the public is very much the same Jack Layton as he is personally. He was upbeat, he encouraged me to keep going.
"One of the difficult things for me was that he would often apologize to me. He would sort of say, 'I'm so sorry that you have to go through this,' which is so unbelievable to me that he would do that."
McGrath would see Layton weekly, usually on Saturday, at the home he shared with wife and fellow New Democrat MP Olivia Chow.
Her mid-August visit wasn't encouraging. McGrath asked Layton what kind of day it was.
"And he said, '50-50.' So it was a rough day."
This last weekend was better. McGrath arrived at about 3 p.m. for a meeting that included Layton, his wife, and Brian Topp, the party president.
Layton sat in a living room chair wearing a zip-up fleece and sweat pants McGrath had retrieved from Stornoway, the official Ottawa residence where he and Olivia had spent so little time.
"He said he thought that the pain management was going well, he said he was feeling a little bit more comfortable."
There were worrying signs, though. "He didn't look great. He was very thin. He was in some pain at different times."
Still, Layton's voice was actually stronger than at the news conference weeks earlier.
"He was awake and alert and talking, and challenging, actually," McGrath said.
"He was still very politically sharp. We were talking about the caucus retreats and the opening session of Parliament, and he was asking questions.
"I was presenting something to him, and he said to me, 'OK, talk me through what your thinking is on this.'"
Topp was clearly moved by the visit.
"I was struck at the enormous strength that Jack draws from his family, who were all around him and had been all summer," he said in an interview.
"And so Jack was exactly where he wanted to be in his beautiful, meticulously energy-retrofitted house, surrounded by the people he loved the most. And there's no question he was very ill indeed but he was still Jack Layton."
Before leaving just shy of 7 p.m., McGrath had a moment alone with her longtime political ally.
Parting was difficult, because she knew it might be the last time she saw Layton.
"I kissed him, I told him I loved him. I stroked his face, and then I left."
When McGrath's phone rang, she really wasn't prepared for the news.
"I was actually getting ready to go to bed and I got the call saying he's probably going to pass away in the next few hours.
"It's so funny to be so shocked, when you expect something, but you still are."
Layton was surrounded by family when he died in the early hours of Monday.
"He was at home, he was comfortable, it was peaceful," McGrath said.
"Any of us who've been through a cancer death, it's the way anyone would want it to be, with your family with you, and just a peaceful passing away."