"Is this the part where I can plead the fifth? It's been years, honestly. Yeah, years," Lachey replied when asked by a fan about his famous former spouse on Bravo's "Watch What Happens Live."
"Honestly, probably six years. It was like another lifetime ago."
Lachey, 39, and Simpson, 32, married in 2002. They starred in "Newlyweds," an MTV reality series, for three seasons, then divorced in 2005.
Lachey married TV personality Vanessa Minnillo in 2011 and they have a toddler-age son.
Simpson is pregnant with her second child with her fiance, former NFL player Eric Johnson.
Asked if he reminisces about his life with Simpson by watching episodes of "Newlyweds" on DVD, Lachey laughed and said: "I have not. As you can imagine, that's not popular viewing in my house. You know, we don't sit around as a family and watch the old DVDs."