Born in 1704 to an unknown woman and William Allen I, a prominent and successful merchant in Philadelphia, Pennsylvania, William Allen II spent much of his adolescence in England where he received an extraordinary education and rose in acclaim. For education in common law, Allen II attended Middle Temple, an Inn of the Court in London that calls barristers to the bar, beginning in 1720. He took classes at Clare College of Cambridge University, where he made lifelong connections. Through his legal education and his father’s connections to the merchant class Allen II grew in prominence until the outbreak of the American War for Independence.

Allen I died around 1725, prompting Allen II to return to Philadelphia to take over his father’s business affairs. This was his avenue toward his reputation as one of the wealthiest men in Philadelphia and his involvement in social affairs. He used his prosperity for land acquisition, particularly for hunting grounds in Northampton Town (now in Lehigh County), and for helping those around him. Concerning his philanthropic activities, in 1749 Allen II donated funds toward the creation of the Academy and College of Philadelphia (now the University of Pennsylvania). Of his sons, Andrew, James, and John graduated over the following decade. According to historian E. P. Richardson, in the early 1760s Allen II financed the sugar transportation business of his son John and Joseph Shippen. Richardson also indicates that Allen funded Benjamin West after recognizing the artist’s capabilities and not wishing to see his skills flounder for lack of funds.

Aside from his business and philanthropic ventures, Allen II boasted an impressive career in lawmaking and adjudication of laws in the Province of Pennsylvania. His foray into politics began with serving as Councilman of Philadelphia from 1727 to 1730. From this position, he leveraged himself into the role of a Justice of the Court of Oyer and Terminer—criminal law—of Bucks, Chester, and Philadelphia Counties and the City of Philadelphia (1730) as well as a Justice of the Peace (1730-1750) for Philadelphia City and County. Politically, he served three five-year stints as an Alderman of the City of Philadelphia beginning in 1730, 1736, and 1751. In the intervening years, he was the city’s Mayor (1735-1736) and Recorder (1741-1750). Most notably, he was nominated and then executed the duties of Chief Justice of the Supreme Court of the Province of Pennsylvania from 1750 to 1774. One of the most important cases to come across his bench during this period was the boundary dispute between the Penns and Lord Baltimore. In December 1767, Allen II and his colleagues adjudicated the decade-long case that established the Mason-Dixon line. This boundary physically shaped Pennsylvania and later became a significant demarcation between free and slave states after Northern states abolished slavery. This case, among many others that Allen II cleared from docket backups, helped establish the foundation of common law in the colonies. Notably, Allen procured, designed, and financed the construction of Independence Hall, which was the venue of his justiceship, the boundary case, and the signing of the Declaration of Independence.

On the eve of the American Revolution, Allen II possessed a nuanced political stance on how colonists were being treated and how the Crown should proceed. In 1774, he resigned from his Supreme Court judgeship. According to primary source material from Benjamin Chew, he did so to stymie the Crown from pursuing further heavy-handed actions against the colonies and he lobbied for a more just approach. While sympathetic to colonial resistance to the Crown's taxation policies, he believed the proper redress could and should come from the Crown. In 1775, when war erupted in the colonies, he moved to England and created an estrangement with his family, who stayed in Philadelphia and the surrounding area in eastern Pennsylvania.

Despite being away in England during the war, Allen II attempted to provide counsel and advise his family. Evidence of his influence appears within James Allen’s personal journals. Within them, James recounts signing a legal document in Philadelphia promising to not impede the Continental Army or espouse sentiments against the revolutionary cause. While James seemingly takes his father’s advice from England, in November 1777 his brother William Allen III apparently ignored all his father’s warnings, as he "[rose] a regiment under [General] Howe," an action which James believed Allen II would not consent to "as he looked with abhorrence to the thought." At the same time, however, James noted that he couldn’t imagine Allen III proceeding against their father’s wishes. From this excerpt, it appears that Allen II disapproved of engaging in the war at all, though his son, Allen III, was clearly a loyalist.

In February 1778, Allen II returned to Philadelphia. Some sources suggest his purpose was to bury his son John. He lived at his Mount Airy estate until his own death in 1780. Despite his perceived lack of support for the American cause, various landmarks on the eastern side of the state commemorate him and his life. Northampton Town (renamed Allentown in 1838) and its high school (opened in 1858) were named after him. Although his mansion was demolished in the 1840s, the Philadelphia neighborhood where his estate was located is still known as Mount Airy. William Allen II provides an excellent example of an influential colonial person conflicted with loyalty to the British crown and empathy for colonists seeking reprieve from their government’s perceived injustices.