Speaking to Piers Morgan for an upcoming episode of ITV's Life Stories programme, the Labour leader, 48, revealed the pain of watching his mother Jo's health deteriorate as he closed in on being elected to Parliament.
Jo, a life-long Labour supporter, suffered from Still's disease - a rare form of arthritis which can destroy the joints.
She died in 2015 - just weeks before Sir Keir was elected MP for Holborn and St Pancras.
In quotes carried by the Mail on Sunday, Sir Keir said: "It was really tragic. She would have loved to have seen that. But she was so ill by then.
"She couldn't move, she couldn't use her hands, so she had to be fed.
"She couldn't speak, couldn't communicate. I would have loved her to have been there but she was in a terrible way. In a terrible, terrible place."
When asked what he would have liked to have said to his mother if he had the opportunity, Sir Keir responded: "I love you."
In the interview, which airs at 9.30pm on June 1, Sir Keir also said a government under his leadership would make improving education and creating an economy that deals with insecurity and inequality its top priorities.
He said: "These are the big ticket items coming out of the pandemic. They were the big ticket items before but now they are just more brutally exposed."