dayton was born in basking ridge, new jersey, to farmer joel dayton (1776–1833) and nancy (lewis) dayton (1787–1866). his father worked as a farmer and mechanic, and was not well off, but the extended dayton family was long prominent in new jersey. william l. dayton was the grand-nephew of elias dayton and second cousin of jonathan dayton. he graduated from the college of new jersey (now princeton university) in 1825. he then studied law with peter dumont vroom, was admitted to the bar in 1830, and became an attorney in freehold.