She was cast on The Lone Ranger in 1953, and then again on that popular series in 1955. Coates was cast in 1955 as Madge in the CBS sitcom Professional Father. In 1955, she portrayed Medora De More in the two-part episode "King of the Dakotas" of the NBC western anthology series Frontier. In 1956, she was cast in the episode "God in the Street" of another anthology series, Crossroads, based on the lives of American clergymen. That same year, she appeared in a second religious drama, This Is the Life, as Betty in the episode "I Killed Lieutenant Hartwell." She was also cast in 1956 as Marge in the episode "Web Feet" of the military drama Navy Log. She guest-starred in David Janssen's crime drama Richard Diamond, Private Detective.
Coates played a strong-willed Lois Lane in the first twenty-six episodes of Adventures of Superman, in which she was given equal billing with George Reeves (insisted upon by Reeves), even for episodes in which she did not appear. Her powerful "damsel in distress" scream was used to good effect in several episodes. After shooting wrapped on the first season, the Superman producers suspended production until they found a national sponsor. When in 1953 it was possible to film more Superman episodes, Coates was already committed to another series. Noel Neill, who had played Lois Lane in two Columbia Superman serials, in 1948 and 1950, replaced Coates. With the death of Noel Neill on July 3, 2016, Coates became the last surviving regular cast member from the Adventures of Superman TV series.