Johnstone began to foster his interest in the arts at a young age and continued to create throughout his life. He worked within a wide range of media, including pastels, oils, acrylics, collage, and sculpture. Some of his works can be seen in the collections of institutions such as the Everson Museum, Syracuse University, and the Munson-Williams-Proctor Institute.
As an artist Andrew Johnstone did not limit himself to the use of one media, he explored different techniques and materials. He was a painter, a sculptor and he also made mosaics. Most of his paintings fall under the movement, of abstract expressionism, this is certainly visible in his later works.