A native of the Horicon Marsh area of Wisconsin, Terrill grew up in an area rich in wetlands and natural areas. He was attracted to the variety of migrating birds that came and went through this area, and his love of nature blended with his love of the visual arts. Terrill attended the University of Wisconsin-Madison earning degrees in art and wildlife ecology. Upon graduation, he pursued a career in painting and was an understudy of museum curator and Natural History artist, Owen Gromme, for seven years. Terrill was Gromme's only student.
Knaack's paintings have been shown at many of the nations natural history museums, as well as at numerous Leigh Yawkey Woodson Art Museum shows and the national Geographic Society's Explorer's Hall in Washington D.C. Terrill has had numerous one man shows in his career including featured visual artist of the Green Lake Festival of Art. In 1983, Terrill received the Alumni Achievement Award from Wayland Academy and was selected as Honorary Chairperson for Wisconsin Youth Art Month in 1986. He has continued to lend his support to art education programs from high school through the college level