Joel holds both Swedish and American citizenship. His father is American-born and his mother is Swedish-born.
Half-brother of actress Melinda Kinnaman. He has four other sisters.
Joel's paternal grandparents were David H. Kinnaman, from Perry Township, Marion County, Indiana, and Rosalie L. (Mitchell), from Kansas City, Kansas. Joel's father's ancestry includes German, English, Scottish, and Irish. Joel's mother is of Swedish Jewish background (she is descended from Ukrainian Jewish immigrants to Sweden).
In 2016, he married model/tattoo artist Cleo Wattenström.
Was in a short-term relationship with Jane Timglas and Olivia Munn.
Friends with fellow Swedish actor Gustaf Skarsgård for a long time.
Kinnaman has a condition called "Pectus excavatum," a congenital deformity of the anterior thoracic wall in which the sternum and rib cage grow abnormally, producing a sunken appearance of the chest. For this condition, Joel underwent corrective surgery that involved the insertion of 2 curved metal bars behind his ribs that extended across the width of his chest. When rotated, the curvature of the bars moved outward against the rear of his sternum forcing a greater gap between his heart and chest, filling out the characteristic sunken chest cavity associated with the condition. Once the desired chest contour was determined, the bars were attached to his ribs. For his role in Altered Carbon (2018) Joel was still fitted with these metal braces.
During an appearance on The Late Show with Stephen Colbert (2015), Kinnaman revealed that he went to the same high school with Alexander Skarsgård and Noomi Rapace in Stockholm.