He was a political and religious radical who prospered during the years of the Parliamentary ascendancy and was an early settler of New England in North America.

His father, William Rainsborough, was a captain and Vice-Admiral in the Royal Navy, and later Ambassador to Morocco (at which time he declined an Hereditary Knighthood).

He married a woman named Margery Jenney of Suffolk upon his return and almost immediately went to serve in the Navy.

In the New Model Army, he was a Captain serving for Colonel Thomas Sheffield as seen in the Naseby order of battle.

William's brother, Thomas Rainborowe, was more fiery in his Leveller speeches, and he was murdered by Royalists in a bungled kidnap attempt in Doncaster in October 1648.

William led the funeral, and a pamphlet called The Second Part of Englands New Chaines Discovered of that year discussed Rainsborowe's attempts at finding justice and the resistance of the upper classes.

William Rainsborowe's cornet, according to the Dictionary of National Biography, was, during this time, a depiction of the severed head of Charles I and the motto salus populi: suprema lex ("let the good of the people be the supreme law").

He hosted Ranters meetings, and he was arrested for paying for the publication of Laurence Clarkson's The Single Eye (1650).

A political radical selling a great store of pistols just in the midst of a change from republicanism to a restored Monarchy and Crown was extremely provocative.