Lord of Killeen jure uxoris, by right of his wife. Succeeded to the Gerardstown estates and was in possession from at least 1351. In 1361 he was knighted in the field by Lionel, Duke of Clarence & Earl of Ulster, who was the King's son and governor of Ireland, during an autumn campaign against the O Byrnes. In 1375 he was fined 100 shillings for not attending a parliament at Kilkenny, and in Jan 1376 he was summoned to a parliament in Dublin. In 1378-9 he was seneschal of Meath. He was found short £100 in his accounts and imprisoned in the Marshalsea debtors prison. On 26 Dec 1380 the King ordered his release from prison on petition of his son, Luke. The followingyear Luke gave recognizance of £200, a very large sum then, on behalf of him and his father, which seems to have cleared the debt owed by Walter and reinstated him. He was again summoned to a parliament in 1382. On 10 Jun he was appointed a justice to investigate felonies, breaches of the peace, etc, in Meath.
He married 1st Joan Tuite, Lady of Killeen, daughter and heiress of Sir Richard Tuite, Lord of Killeen. Joan had livery of the estate on 24 Jun 1351, thereby returning the manor of Killeen to the Cusack family. She separated from Sir Walter and obtained a divorce from the Bishop of Meath on the grounds that beofre marriage to her, Sir Walter had cohabited wtih a woman related to her in the third degree of consanguinity. Although Sir Walter made no move at the time, and Joan married Sir Henry Ferrers, on a visitation of the Archbishop of Armagh he suggested to this prelate that his marriage to Joan had been valid. The Archbishop, without examining Henry and Joan, declared her former marriage to Sir Walter valid and ordered Joan to go back and live with Sir Walter as a wife. Against this order, Henry and Joan appealed to Pope Urban V in 1364. The Pope called for a report on the case in a letter addressed to the Bishop, Dean and Archdeacon of Clonmacnoise. Joan however did not return to Sir Walter, and a violent struggle over her inheritance took place. The Justices arrested Henry Ferrers and Walter Cusack and forbade them from doing anything against each other except by the law of Ireland. Nevertheless, Sir Walter, with the support of Sir James le Hide, Simon Cusack, son of Sir John Cusack, John Rudipak and three of the Petits, ejected Henry and Joan from Joan's estate of the annual value of 600 marks by armed force, without any process of law, took goods away to the value of 500 marks, assaulted and wounded Sir Henry Ferrers and besieged him in the manor of Clonee until he was rescued by Lionel, Duke of Clarence.
Sir Henry and Joan were so threatened in life and limb that they dared not stay in those parts to prosecute their rights against Sir Walter. Henry and Joan, their men, servants, lands, rents and possessions were therefore taken into the special protection fo the King for one year by letter from Westminster addressed to Lionel, Duke of Clarence, dated 26 Oct 1366. When Joan died not long after this, his eldest son by her inherited Killeen as heir to his mother. He married 2dly Elizabeth. In 1381 Peter Rice of Rystown recovered his father's manor of Rystown in the court of the liberty of Trim by writ of mon d'ancestor against Sir Walter Cusack and Elizabeth his wife, who were fined for unjust detention. On 24 Sep 1388 Sir Walter was made custodian of the manor of Clonee on surety of his son, Luke,, and John Bermingham, who was married to his daughter, Joan Cusack. In 1393-4 Sir Walter and Lady Elizabeth levied a fine. He died before 1402.9