The son of a publisher initially worked in his father's operation, before he got small roles on stage and in film because of his good looks. His title role as a student of Heidelberg in the UK brought him to the attention of Warner Bros. Studios in Hollywood, which immediately took him under contract and gave him the son of a British lord in Give Me Your Heart a first major role. However, his breakthrough gained Knowles at Errol Flynn and Olivia de Havilland page in The Treason of Surat Khan and again two years later with the two in Robin Hood: Prince of vagabonds as Will Scarlet. There were other films at the Flynn's side. Although it several times to him - including in and the four of Love - managed to relax his rival the bride at the end, he remained as a film actor in the shadow Flynn.
In 1939 he ended his collaboration with Warner Bros. and began working as a freelance actor, but rarely came over and played supporting roles also infrequently in films, which is why he mostly worked in theater. He was also in World War II as an instructor. The tall Knowles, in film criticism often referred to as "the poor man's Errol Flynn ", was occupied on stereotypical roles because of his aristocratic features, the distinguished Schnauzbartes and his British origin. The crux of his acting career describes a limerick from the forties:
"How pleasant to know Patric Knowles Who is the kindest of souls But being handsome and a British swell Nobody Expects him to act very well Which is why he never gets good roles. "
" How pleasant it is to know Patric Knowles who has the kindest soul but as attractive and a spiffy Brit nobody expects him to act well this is the reason why he never gets good roles "
Supporting roles as a gentleman, he also played a detective in the cheap horror movie production The Strange Case of Dr. RX and a miner in John Ford's drama Green Was My Valley. Increasingly, however, he also played negative characters: Knowles played a mad scientist in the horror film Frankenstein Meets the Wolf Man, a volatile and pursued by Robert Mitchum thief in the thriller The Big Steal. With Claudette Colbert, he played in three returned home (1950 ) the main roles. In the 1960s he was about, directed by Andrew V. McLaglen in the western The Way West and Chisum front of the camera. One of his last roles was that of Lord Louis Mountbatten in the wartime drama The Devil's Brigade - again directed by McLaglen. Beginning of the 1970s had his career fade away definitively around 125 film and television appearances.
1960 Knowles received a star on the Hollywood Walk of Fame. After the end of his acting career, Knowles gave away other pursuits such as painting. He was also the author of a novel titled Even Steven (1960) and wrote in 1990 a pamphlet called Rebuttal for a Friend, in which he stated that Errol Flynn had never been a Nazi spy, after such rumors had arisen in the decades before. Knowles died on 23 December 1995 at the age of 84 in Woodland Hills near Los Angeles of a brain hemorrhage, a few months after the diamond wedding with his wife Enid. They had two children.