A heavy smoker, Rennie had respiratory problems for many years. During the run of the Broadway comedy "Mary, Mary" in the early 60s, Rennie had to visit the hospital several times; this didn't prevent him from being hired to replay his role in the film version, Mary, Mary (1963). His voice was dubbed in the movie The Battle of El Alamein (1969) ("The Battle of El Alamein") due to his chronic health problems. Similarly, his own voice is not heard on the English-language dialogue track of his final film, Assignment Terror (1970) ("Assignment Terror").
Appeared in The 13th Letter (1951) for director Otto Preminger, who later named Rennie as a third party to his counter-suit of adultery against his wife during divorce proceedings in 1958.
Close friends during his 20th Century-Fox years with Tyrone Power and appeared in a few of his pictures, including The Black Rose (1950) and I'll Never Forget You (1951).
Did not begin to actively pursue acting until age 29. Living a rather wanderlust life prior to this, he was once a car salesman, but quit after a year when he couldn't sell even one car. Was also a manager of a rope factory for his uncle.
Graduated from The Leys (a private school in the city of Cambridge). Despite his lean build, he excelled in sports (rowing, fencing, cricket, boxing, wrestling, swimming).
He had a second son, John Marshall Rennie, with longtime companion Renee Gilbert Taylor. Professionally, his son went by John M. Taylor.
His early career was interrupted by war service. He was a flight instructor for over two of those years.
His son David Rennie, is now a UK High Court judge on the Lewes, Sussex circuit.
In his best known film, the classic sci-fi The Day the Earth Stood Still (1951), a scene was cut that showed his even-tempered alien character Klaatu in an emotional state.
Lived his final years in Geneva, Switzerland and died at his mother's home in England while visiting because of his brother's death.
Michael Rennie joined the Royal Air Force in 1941, training as a fighter pilot in the United States under the Arnold Plan. While at Napier Field, Dothan, Alabama for his advanced flight training he was asked by fellow trainee, Scotsman Jack Morton, what he did in civilian life. Rennie told Morton and the other pilots gathered around that he was a movie actor. They stared at him in disbelief then broke out in a chorus of laughter. A couple of nights later Rennie and his classmates went into town to watch a movie, Ships with Wings (1941). Not long into the movie, and much to the surprise of those seated with him, Michael Rennie appeared on the screen as Royal Navy Pilot "Lt. Maxwell".
Once married to British actress Maggie McGrath, who later went by the stage name of Maggie Rennie. They had one son, David, born 1953.
Rennie was connected with the Titanic twice on screen. In 1953, he was the uncredited, off-screen narrator for Jean Negulesco's film Titanic (1953). Thirteen years later, in 1966, he played the role of Titanic Captain Edward J. Smith (misidentified in the credits as Malcolm Smith) in the pilot episode of the TV series "The Time Tunnel" (1966). Ironically, this episode used tinted stock footage from the earlier film to tell the story of two time travelers who found themselves aboard the doomed ship.
Son of James Rennie (not the actor), who operated a century-old wool mill, and Edith Dobby Rennie. His great-great grandfather, named John Rennie, designed and built New London Bridge.