Born in Columbia, Mississippi, at age 4 she moved with her family to Covington, Louisiana. She attended high school and junior college at Gulf Park College in Gulfport, Mississippi (now the Gulf Park campus of the University of Southern Mississippi), then finished college at Northwestern University in Illinois, appearing in college plays and receiving her degree from Northwestern's School of Speech in 1948.

After brief modeling and radio experience, Dow was spotted by a talent agent and cast in a television show in February 1949. Shortly after that exposure, Universal offered her a seven-year contract. Dow made nine films, most notably as Nurse Kelly in Harvey (1950), starring James Stewart, and co-starring with Best Actor Oscar nominee Arthur Kennedy in Bright Victory (1951). After being featured in several crime dramas, Dow had starring roles in two 1951 family films, Reunion in Reno and You Never Can Tell.

She retired after three years in the Business to marry Walter Helmerich III, an oil driller from Tulsa, Oklahoma, in 1951. He became President of his family's Business, Helmerich & Payne. They were married for 60 years, until his death in 2012. The couple had five sons. She became an active supporter of libraries and other charitable activities.

The Peggy V. Helmerich Distinguished Author Award, an award given annually since 1985 to a distinguished author by the Tulsa Library Trust, is named in her honor, as is the drama school at the University of Oklahoma and the auditorium at Northwestern's Annie May Swift Hall.