At a young age Korthals was encouraged by his uncle, Marie Henri Mackenzie, to become an artist. However, a professional artistic education was out of the question and he became a bookkeeper. In the year 1942 he decided to give up this job and focus entirely on his art.
He produced townscapes, portraits, landscapes and still-life paintings in various styles.
Korthals also became well known for his book and calendar illustrations.