He was the eldest son of John Foster, Member of Parliament for Dunleer, and his wife Elizabeth Fortescue, youngest daughter of William Fortescue of Neuragh, who was a member of the Fortescue family which later held the title Earl of Clermont.
He attended the school in Dublin run by Thomas Sheridan, the friend of Jonathan Swift and grandfather of Richard Brinsley Sheridan.
The Fosters had come to Ireland from Cumberland in the previous century, and slowly acquired lands and political influence in Louth. He was Chief Baron of the Irish Exchequer 1766-1777. Prior to appointment to the Bench he represented the family constituency of Dunleer in the Irish House of Commons from 1738 to 1761 and subsequently Louth from 1761 to 1767.
He matriculated from the University of Dublin in 1722 and took his degree of Bachelor of Arts in 1726.
He entered Middle Temple in 1726 and was called to the Irish Bar in 1732. As an orator (which was a much-prized skill among the Irish public figures of his time) he was badly thought of, being described as "slow, sleepy and charmless".
Foster was not regarded as the most outstanding lawyer on the Irish Bench in his lifetime, but it has been argued that he was its most gifted member overall, with his various interests in law, politics, trade and agriculture. If he was quickly forgotten, this may be because his reputation was eclipsed by that of his even more gifted son, John.
As a member of Parliament he worked tirelessly to promote the interests of the manufacturers of Irish linen. There is no reason to doubt the sincerity of his belief in this cause (which fits well with his known interest in the improvement of agriculture), although Ball rather cynically notes that it brought him rich rewards, including a gold box.
