Maheu's son, Peter, told the Los Angeles Times that his father died Monday of congestive heart failure at a Las Vegas hospital. He was 90.
Maheu first worked for Hughes in 1955, a year after he set up his detective agency. While his early jobs included following actress Ava Gardner, Hughes's one-time girlfriend, he later became Hughes's link to the outside world as the billionaire became mentally ill.
"A lot of people wished they were Howard Hughes, but Howard Hughes wanted to be Bob Maheu," Pat H. Broeske, author of a Hughes biography, told the Times. "He was very envious of Bob in many ways. He liked manly men."
During the 15 years Maheu worked for Hughes, they communicated entirely by telephone and written memo.
Maheu helped Hughes become involved in Las Vegas, advising the billionaire to "buy a hotel" if he needed a place to live. Hughes bought the Desert Inn, where he was staying at the time, and went on to acquire several more casinos.
Celebrity Families of 2014 [PHOTOS]