Following his education in Hengelo, Haanstra became a schoolteacher in Markelo. He combined a job in Arnhem with evening classes at the Academy for Fine Arts (later ArtEZ) in the same city. After giving drawing lessons at various secondary schools and at the Teacher Training Academy in Heerenveen, in 1963 Haanstra was invited to teach at the Minerva Academy in Groningen and in 1964 went to live in Vries. For a period of five years, from 1969 to 1974, he interrupted this employment at the Academy to earn his living as an independent artist.
He worked in both abstract and figurative styles, with a particular focus on the landscape and the livestock on it. As a monumental artist, he created mosaics and murals. Folkert Haanstra was a member of the Drents Schildersgenootschap (Drenthe Artists’ Society).