The revered method actor, who is up for his third Oscar next month for his portrayal of U.S. President Abraham Lincoln, is intensely private, and even quit acting in the 1990s to become a cobbler, before returning to the big screen in 2002's Gangs of New York.
And his Hollywood persona was such a mystery to his sons, his second child Ronan presumed he worked in construction.
He tells the Daily Mirror: "They didn't even know what I was doing until a couple of years ago.
"In fact, my 14-year-old boy was asked what I did and he said: 'I think he's in construction'. That's how much they know!"
Day-Lewis is dad to two boys with his wife Rebecca Miller, Ronan and Cashel, while he also fathered a son named Gabriel with his ex-girlfriend, French actress Isabelle Adjani.