The Anglican leader described experiencing moments of hopelessness, during an intimate interview with Alistair Campbell at GQ magazine.
Asked whether he becomes depressed, Archbishop Justin answered: "I think if you had asked me a year ago I'd have said no, and ten years ago I would have said absolutely not.
"But what was that phrase Churchill used? 'Black dog'. There is an element of that. I think as I am getting older I am realising it does come from time to time.
"I have those moments - you would know this - when objectively everything is fine, but you think you are, beyond description, hopeless."
Archbishop Justin told Mr Campbell his response was to talk to God about his feelings. The 61 year old explained he had never sought professional help for the so-called 'black dog'.
His frank comments come two months after his daughter Katherine Welby revealed how she had kept her own mental ill-health a secret from him.
The 31 year old, who was diagnosed with depression aged 19, told The Sunday Times that, despite her father's public role in the Church, she decided initially not to seek his help.