Guy acquired high military honors in the martial reign of Edward I, distinguishing himself at the Battle of Falkirk, for which he was rewarded with extensive grants of land in Scotland, at the seige of Caerlaverock, and upon different occasions and also beyond the seas. In the reign of Edward II (1306-1326) he likewise played a very prominent part. In 1310 his lordship was in the commission appointed by parliament to draw up regulations for "the well governing of the kingdom and the king's household," in consequence of the corrupt influence at that period by Piers Gaveston, in the affairs of the realm, through the unbounded partiality of the king; and in two years afterwards, when that unhappy favorite fell into the hands of his enemies, upon the surrender of Scarborough Castle, his lordship violently seized upon his person, and after a summary trial caused him to be beheaded at Blacklow Hill near Warwick. The Earl's hostility to Gaveston is said to have been much increased by learning that the favorite had nicknamed him "the Black Dog of Aedenne." For this unwarrantable proceeding his lordship, and all others concerned therein, received in two years the royal pardon, but he is supposed to have eventually perished by poison, administered by the partisans of Gaveston.
The Earl married Alice, daughter of Ralph de Toni, son (by Alice de Bohun) of Ralph de Toni of Flamstead, County Herts, and had Thomas, his successor, John, Maud, Emma, Isabel, Elizabeth and Lucia. This great Earl of Warwick was, like most of the nobles of his time, a munificent benefactor to the church, having bestowed lands upon several religious houses, and founded a chantry of priests at his Manor of Elmley. His will bears date "at Warwick Castle" on Monday next after the Feast of St. James the Apostle 1315, and by it he bequeaths to Alice, his wife, a proportion of his plate, with a crystal cup, and half his bedding; as also all the vestments and books belonging to his chapel; the other moiety of his beds, rings and jewels he gives to his daughters. To his son Thomas his best coat of mail, helmet and suit of harness, and to his son John his second suit of mail, etc., appointing that all the rest of his armour, bows and other warlike "provisions" should remain at Warwick Castle for his heir. His widow married 2nd William la Zouche of Ashby, County Leicester. The Earl died at Warwick Castle August 12, 1315, succeeded by his eldest son, then but two years of age.