After traveling to Europe where he met many European astronomers, Shapley made a stop in Kansas City to marry his wife, Martha Betz Shapley, in April 1914. The same year, Harlow was offered and accepted a position at Mount Wilson Observatory in California. Martha assisted her husband in astronomical research at Mount Wilson and later at the Harvard College Observatory. She authored or co-authored numerous articles on eclipsing stars and other astronomical objects. Harlow and Martha had five children together: four sons and one daughter.