Foster graduated from Harvard College in 1756 and went on to study theology.
In 1761 Foster was ordained minister at Canterbury, New Hampshire, then a rapidly growing frontier settlement. His father had invested in lands in this district and several of his brothers moved there about the same time. For eighteen years Foster continued his ministerial duties in this town and its records show that he was also active in various secular affairs. The last years of his pastorate were embittered by a factional quarrel in the church and in 1779 he was formally dismissed.
Foster now abandoned the ministry and henceforth devoted himself to public service. He was an active supporter of the Revolutionary movement and a member of the Provincial Congress at Exeter in 1775.
From 1779 to 1783 Foster represented Canterbury in the legislature, and was a delegate to Congress from 1783 to 1785. The records of the latter body show that he was faithful in attendance and active in the performance of miscellaneous routine duties. In 1784 he was appointed a judge in the court of common pleas and continued his duties for four years. This court did not as yet require the services of men learned in the law, and justice could be satisfactorily administered by persons of integrity and common sense.
Following the adoption of the Constitution Foster was elected to the First Congress, was defeated for reelection, and in the interim between March 1791 and his election to the Fourth Congress in 1794, devoted himself to New Hampshire affairs, serving in the Senate 1791-93, one term as president of that body, and also as a member of the important constitutional convention of 1791 - 1792.
His second period of service in Congress covered the years from December 7, 1795 to March 4, 1803. His health began to fail during his last term and he retired from active politics in the latter year. Abiel Foster died on February 6, 1806, in Canterbury, New Hampshire.
Abiel Foster was a deputy to the Provincial Congress, a member of the Continental Congress, (1783 - 1785) and judge of the court of common pleas of Rockingham County, (1784 - 1788). In 1790, he was elected as a Pro-Administration candidate to the First Congress, serving until 1791.
After his term, Foster was a member of the New Hampshire State Senate, (1791 - 1794) and elected as a Federalist to the Fourth Congress and to the next three succeeding Congresses, serving (1795 - 1803).
Abiel Foster was never active in debate but consistently supported Federalist policies, and he was a dependable member of that party.
William Plumer, whose judgments of his contemporaries were inclined to harshness, describes him as "more distinguished for practical than theoretical knowledge’" but possessed of honesty and sound judgment. Furthermore, "he never avoided voting upon any question. "
On May 15, 1761, Abiel Foster married Hannah Badger, who died in 1768.
In 1769, Foster married Mary Rogers, daughter of Samuel Rogers and Hannah Wise of Ipswich, Massachusetts. They had eight children.