Oakley began his study of illustration with Howard Pyle in 1902, working with him for three years, both at Pyle's winter studio on North Franklin St. in Wilmington, Delaware,[1] and at his summer studio in Chadds Ford, Pennsylvania,[2] which was situated in the old mill that now houses the Brandywine River Museum.

Almost half a century later, Oakley described his first day with Pyle in an address he delivered at the Free Library of Philadelphia, on the occasion of which he also presented his collection of Pyleana – drawings, prints, books and other items, including letters and sketchbooks – to the Free Library: There we four - my new cronies - Allen Tupper True, George Harding, Gordon McCouch and I - made our first sketches from a model, and our efforts were frightful to behold!

Thornton Oakley never learned the nuances of color but had an ingrained predilection for the primaries, red, yellow and blue.

In the 1951 address he delivered at the Free Library of Philadelphia, referred to above, he said, "We never heard one word from our beloved teacher concerning tools and methods.

Among those pictorial artists who have been definitely connected with making clear statements, pre-eminently stands Howard Pyle.

[9] Oakley had previously expounded his own philosophy of illustration as a "pictorial making clear" in an entire essay on that subject in The American Magazine of Art in 1919.

In 1932, in recognition of his artistic services to France, the Third French Republic decorated Oakley with the Palmes d'Officier d'Académie, an honor rarely conferred upon foreigners.

In 1943, he published a short monograph[14] as a tribute to his long-time friend and fellow artist, Cecilia Beaux, who had died in the previous year.

Whether a man speak with brush or mallet; pen or note; by utterances, statesmanship, gift, or friendship; whether by his daily routine, business, or by whatever activity to which his existence may be called; — if he thrill the inmost being, lift with visions toward the stars, reveal the beauty of ennobling life, then, and then alone, may he be named by that most inspiring of all titles, Artist.

[16]The Brandywine River Museum maintains a collection of Thornton Oakley memorabilia, including news clippings, business correspondence, sketchbooks, personal diaries, and other materials, all donated by his daughter Lansdale in 1981.