[4] From 1920 to 1959, Johnston maintained a private internal medicine practice in Des Moines, Iowa, specializing in women's health and pediatrics.

[1] During the early 1920s she was involved with baby clinics held at Iowa State University under provisions of the Sheppard–Towner Act.

[9] She was elected the ninth national president of Altrusa International in 1928[10] and 1929, and stayed involved with the organization for many years after.

[1][4][15] Helen Johnston had spinal surgery in 1962,[16] and left Des Moines in 1964 to live with her friend, Mrs. Arthur Turner, in Sun City, California.

[1][17] Her home in Des Moines, built in 1938, is still a striking example of modern architecture in that city, now known as the Helen Johnston House.