The Beatles legend's only son was 20 years old when his mother Linda lost her cancer battle in 1998, and he admits he drifted from his famous father and began to listen to heavy rock in the wake of her passing.
But it was only when the rocker met Mills in 1999 that he started losing contact with his father and siblings and began taking Class-A narcotics.
He tells Britain's Daily Mirror, "It was a difficult time. Dad was married to Heather and we didn't get on. I don't have a problem with Heather now because she is (my half-sister) Beatrice's mum but at the time it was difficult for me, I was so into Kurt Cobain and this mentality so I didn't really like her. It was very difficult, I had my own issues.
"We drifted quite far apart, it was a difficult period. It was a coming of age for me. I had to strike out on my own and I needed a bit of time to grieve my mum's death... The estrangement was partly because of Heather. It was difficult because of mum's death and then moving on, having a new mother.
"I started getting more into Class-A drugs, ketamine. I wasn't doing heroin but it's practically like heroin... but not quite the same.
"It's not that big a deal... but it is at the same time. It was my mistake. It wasn't a problem, I think it was just once, it wasn't that bad. All the family were concerned for me, they were worried. But it got to the point when they were just: 'effin (sic) let James do his own thing.'
"At the time I severed that tie with the rest of the family a little bit which I wish I hadn't done now."
James and his father eventually made amends and MCCartney divorced Mills in 2008. He is now married to American socialite Nancy Shevell.