The admissions register at Lincolns Inn Library shows that William Pitt, the second son of the Right Hon. William Pitt, Earl of Chatham, was admitted on 28 January 1777; at the time of his entry, he held an MA from Pembroke College, Cambridge . This information corresponds with a letter from Pitt's mother Hester, Countess of Chatham to Mrs. Pitt, dated 30 January 1777. In the postscript of her letter, the Countess of Chatham proudly boasts that, "William is gone this morning to keep his Term in Lincolns Inn...He is not quite 18."  Perhaps he was entered in the register on the 28th, but did not make his way there until several days later.
By the end of 1779, William was residing at Lincolns Inn the majority of the time. He wrote to his mother on December 18, 1779, informing her of his address:
“My Time has been pretty much taken up by establishing myself in my new Quarters here, and by my frequent Attendance on the debates…My Abode is for the present known by the name of Stone Buildings, Lincolns Inn." 
The Black Books at Lincolns Inn indicate that he was called to the Bar on 12 June 1780. Other interesting and less well-known information regarding William's association with Lincolns Inn is that he was made a bencher of the Inn on the 28th of November 1782; his coat of arms were put up in the Hall on 14th May 1783, and the Inn received £9 9s 10d from Pitt as rent for his chambers in 4 Stone Buildings.  It is also known that Pitt was made Keeper of the Black Book for 1789, Dean of Chapel for 1790, Treasurer in 1794, Master of the Library in 1795, and Master of the Walks in 1796. Lastly, the Black Books record that Pitt made a payment of £2 13s 4d for his chamber, supper, and purse on 6th November 1795.  After that date, it seems nothing further is recorded of Pitt's association with Lincolns Inn.
1. This information was obtained from Robert Athol, the Archives and Records Manager at Lincolns Inn Library.
2. Hester, Countess of Chatham to Mrs. Pitt. 30 January 1777. British Library Add Ms 59490, f. 38.
3. William Pitt to his mother Hester, Countess of Chatham. 18 December 1779, PRO 30/8/12, f. 127.
4-6. I am indebted once again to Robert Athol, the Archives and Records Manager at Lincolns Inn Library.