Donald is a married man and he is married to Jill Knuth, however, there is no more information about when he was married and how many children he has. There is also no more information about his extra marital affairs and the case of these sweet couple getting divorced. In 2006, he was diagnosed with prostate cancer. He underwent surgery the same year and started "a little bit of radiation therapy... as a precaution but the prognosis looks pretty good"; he stated these things in his video autobiography.
Donald was in an official group of people to write a book on computer programming language compilers. While he was on this project, he concluded that he could not adequately create the topic without first developing a fundamental theory of computer programming, which became The Art of Computer Programming. He published the first volume of his book in 1968. Prior to publishing the 1st volume of The Art of Computer Programming, he left Cal-tech to accept employment with the Institute for Defense Analyses' Communications Research Division, then situated on the Princeton University campus, which was performing mathematical research in cryptography to support the National Security Agency. He earned a huge salary being a scientist and a professor at various institutions. However, his real time net worth is unknown to the press.