The premise of "Who Wants to Be A Millionaire" is that if a contestant correctly answers several questions of approximately increasing difficulty, s/he will become a millionaire.
However, even if they correctly answer the $1,000,000 question, they still have to pay taxes on that (which is quite a lot), and 3/4 of the money comes in equal installments over 20 years, an annuity (which, if Powerball figures are a guide, have an initial cash value of about half the payments' total). Source: Official rules.
However, it is possible that some contestant came into the show with enough assets saved up so that a prize check put their total balance up over $1MM.
Are there documented instances of the show actually making someone a millionaire?
To clarify, the question refers primarily to the US version of the show.