The Lamberton surname is recorded in one source as having a close association with the ancient Barony of Kilmaurs, Ayrshire and the Lands of Lambroughton.

He was appointed Bishop of St Andrews in 1298 by Pope Boniface VIII in succession to William Fraser.

When being consecrated as bishop, Lamberton went to France to build support for Scotland in the French Court and Papal Curia.

As early as June 1298 Lamberton had won victories for the Scottish cause; his activities led both the King of France Philip IV and Pope Boniface VIII to pressure Edward I to halt his attacks on Scotland.

In a letter to Scottish leaders dated 6 April 1299 Philip IV commended Lamberton's efforts and declared he would assist Scotland.

Bishop Lamberton's diplomatic abilities were later recognised by the Scottish Magnates when he was chosen as a third Guardian, alongside Robert Bruce and John Comyn in 1299.

After forming close ties with Bruce during his time as Guardian, Lamberton saw him as a potential leader of a fight for independence.

Lamberton and Wishart were arrested and put in irons for their roles in Bruce's coronation and transported to London where they were imprisoned.

During his time in England, Lamberton stayed near the Tweed so that he could maintain connections with the parts of his diocese under English rule.

Lamberton attended negotiations in 1309 as an "English" envoy, and his skill as a diplomat helped him to keep on good terms with both Edward and Robert.

Lamberton along with Thomas Randolph, 1st Earl of Moray served as the Scottish negotiators in 1323 when a thirteen-year truce was agreed to between Scotland and England.

King Robert and the Community of the Realm sent three separate replies to Pope John XXII; the last group's letter, now known as the Declaration of Arbroath, is the most famous.

[7] Bishop William de Lamberton died on 20 May 1328,[1] 18 days after England officially recognized that Scotland was a fully sovereign, independent state.