The original settlers were Welsh-speaking Quakers, led by John Roberts, in an attempt to establish a barony of Wales in Pennsylvania.

[6] In about 1681, a group of Welsh Quakers met with William Penn to secure a grant of land in which they could conduct their affairs in their own language.

The parties agreed on a tract covering 40,000 acres (160 km2), to be constituted as a separate county whose people and government could conduct their affairs in Welsh.

[7] In 1717, the Welsh Friends built a meeting house on a trail made by the Susquehannock in Radnor Township.

The influence of the Welsh, some of whom were forced by heavy taxation to sell their land, waned in the latter half of the 18th century.

The Welsh influence waned in the late 18th century as many left the area due to high taxation.

Lancaster Avenue is the largest and busiest roadway in Radnor Township that is not an interstate, and along it lies many notable places, including Villanova University and Downtown Wayne.

As a result, Radnor Township is today recognized among the collection of Philadelphia suburbs known as the Main Line.

In the 1880s, George W. Childs bought property in the community of Louella in the western part of Radnor Township, renamed the area Wayne, Pennsylvania (after American Revolutionary War hero Anthony Wayne) and organized one of the United States's first suburban developments.

Today, parts of that railroad are still used as SEPTA's Norristown High Speed Line, while others have been repurposed into walking trails.

Radnor Township has a hot-summer humid continental climate (Dfa) and the hardiness zone is 7a.

Some neighborhoods, such as Strafford and Bryn Mawr, are split over municipal lines and are shared by at least one other township adjacent to Radnor.

Beginning in the nineteenth century, an influx of Irish, Italian, and Polish immigrants to the Delaware Valley caused the growth of Catholic neighborhoods in Garrett Hill and Villanova.

[19] According to Sperling's BestPlaces, the largest religious group in Radnor Township is the Catholic Church, represented by the Archdiocese of Philadelphia.

The church, built between 1883 and 1887, is considered to have been the tallest man-made structure between the Delaware River and Lancaster at the time of its construction.

[25] St. David's Episcopal Church, whose graveyard, and buildings begun in 1715, is listed on the National Register of Historic Places.

[26][27] The grave yard hosts many founders including the celebrated revolutionary General “Mad” Anthony Wayne.

[28] Other historic structures in Radnor Township include the Sorrel Horse, a former tavern located on Conestoga Road that sheltered George Washington and Lafayette during the Continental Army's retreat back to Philadelphia from the Battle of Brandywine.

[31] Other companies based in Radnor Township include, J. G. Wentworth, TV Guide magazine, and Airgas.

Valley Forge Military Academy and College contains a boarding school program for seventh through twelfth grades.

The Wildcats are consistently ranked among the AP Top 25, and won the NCAA Division I men's basketball tournament in 1985, 2016, and 2018.

[36] The Cabrini Cavaliers won their first NCAA Division III Men's Lacrosse Championship in 2019, defeating the Amherst Mammoths by a score of 16–12.

[41] The corporate offices of Main Line Health and Thomas Jefferson University's Marcus Institute clinic are both located in the Radnor Financial Center.