After finishing high school she went to Los Angeles to study at UCLA. However, a chance meeting with entertainer Ken Murray in a Hollywood restaurant resulted in her working in his vaudeville show as a chorus girl. She later performed as one of Earl Carroll's showgirls at his Earl Carroll Theatre.
A movie contract with Warner Brothers followed in 1948; she co-starred with George O'Hanlon in the studio's popular Joe McDoakes short-subject comedies. She was briefly married to the series' director, Richard L. Bare, and continued to appear in the films after the marriage had run its course.
She is best known for having played a strong-willed Lois Lane in the first 26 episodes of Adventures of Superman, wherein she was given equal billing with George Reeves, even for a few episodes in which she did not appear. Her powerful "damsel in distress" scream was used to good effect in several episodes.
After shooting enough film for the first season, the Superman producers suspended production until they found a national sponsor. Much later, when it came time to reassemble the cast and crew for new Superman episodes, Coates had already committed herself to another production. Noel Neill, who had played the original Lois Lane in the 1948-1950 serials opposite Kirk Allyn's superman, succeeded her in this series, and became far more identified with the role. (George Reeves is said to have requested Coates to return to the role in 1959, after another production hiatus; his untimely death ended the series permanently.)
In later years, Coates generally made an effort to distance herself from her connection with the Superman series. Coates did have a guest role as Lois Lane's mother, in the wedding episode of the 1990s TV series Lois and Clark.
Her Superman fame has somewhat obscured the fact that Phyllis Coates was one of the screen's most dependable actresses of the period. She freelanced steadily during her Hollywood heyday, appearing in low-budget features, westerns, serials, and the "McDoakes" shorts. Her best-remembered films of the fifties are probably Blues Busters with The Bowery Boys (in which she has a musical number); Panther Girl of the Kongo, a jungle serial in which she starred; and I Was a Teenage Frankenstein.