Sean Hartnett was born in rural upstate New York in 1941. He graduated from Stanford University in 1964 and did graduate studies at Columbia and the University of Perugia in Italy.

He studied sculpture at the Woodstock School of Art in Woodstock, NY, and in 1969 he built a house and studio there. He helped Roman sculptor Raffaele Martini set up the Woodstock bronze foundry.

In 1970 Hartnett moved to Pietrasanta, Italy, and has maintained a studio there ever since, despite visits for work or teaching in Florence, Italy; Austin, Texas and New Haven, Connecticut.

Since 1987 the sculptor has lived and worked in his studios in Pietrasanta and Block Island, Rhode Island. He has shown extensively in Europe and the United States, and his sculptures are included in important collections on both continents.