In addition to being
Thane of
Glamis, Lyon was
jure uxoris Thane of
Tannadyce. As such, he was among the most powerful nobles in
Angus. Upon his marriage to Princess Jean, he acquired the estates of
Kinghorn in
Fife. He was first appointed to a position at the Scottish court sometime prior to 1368 in the reign of
David II, when he was given the responsibility of examining the records of the Chamberlain. He was appointed
Keeper of the Privy Seal upon the accession of
Robert II; from c. 1375 he was the
Keeper of
Edinburgh Castle and was appointed
Lord Chamberlain in 1377, both positions he was to hold until his death. From at least as early as 1367 he started to acquire various properties, from the Earl of Ross in 1367, from John de Hay in 1368. He acquired the thanage of
Glamis from his future father-in-law in March 1372. He was knighted sometime prior to 1377. He was killed (perhaps rather treacherously) on 4 November 1382 during a quarrel with
Sir James Lindsay of Crawford, nephew of the King, near Menmuir in
Angus. ==Marriage and children==