News emerged on the morning of 1 February, a day after Hamilton informed Mercedes team boss Toto Wolff over breakfast, that the seven-time champion would join Ferrari for the start of 2025.
Given the story leaked first, Mercedes and Ferrari did not formally confirm the shock switch until they coordinated their announcements later that evening.
Hamilton says he did not tell his parents until that same day.
Speaking as part of the new BBC podcast series 'F1: Back at Base', narrated by Joseph Fiennes, the driver revealed: "I didn't speak to anybody. I didn't tell my parents till the day of it being announced. So, no one knew.
"I really wanted to do it for myself. Ultimately, I had to find out what would be the best for me."
Hamilton calling time on what will eventually be a 12-year stint at Mercedes has been enabled by a contract extension that he signed last summer.
What appeared to be a straightforward two-year deal was actually for a fixed one-year term with an option for a further 12 months.
This was a lever pushed for by Mercedes, with Wolff saying at the time of the exit news: "We're big boys, we knew that by signing a short-term contract, it could be of benefit for both sides.
"We couldn't commit for a longer period, and he's taken the option to exit."
Hamilton, 39, underlined this element of uncertainty, saying that he did not know what the future would hold or for how much longer he was going to keep racing.
He said: "It happened very fast and basically, I have known Fred [Vasseur, Ferrari F1 boss] for a long time.
"And for me, I was excited for a new year, but not really knowing what the future would hold and not really knowing how long I was going to race for, but knowing that I felt super driven and excited to continue to race.
"I mean, the opportunity just popped up and I was like, 'OK, I've got to think for a second'.
"I didn't have a lot of time to think and I had to just go with my gut feeling and I decided to take the opportunity."
Watch: Hamilton's Shock Move to Ferrari - The Deal Is Done
