In 1036, he received word that his uncle and erstwhile ally, Pandulf of Capua, had attempted to rape his niece, the daughter of his wife's sister and the Duke of Sorrento. He then received the homage of the defecting Rainulf Drengot, formerly a vassal of Pandulf. Thus, Guaimar won the support of the Normans in the Mezzogiorno. In 1037, Guaimar made the politically savvy request of arbitration to both the Holy Roman and Byzantine emperors over the issue of Pandulf's unfitness to rule. Emperor Conrad II accepted the invitation and traveled south in Spring 1038. He demanded hostages from Pandulf. However, the hostages escaped and Capua was promptly besieged. Having taken that principality, he gave it to Guaimar (May), who asked for a title of nobility for his new Norman vassal. This was granted and Rainulf officially became "Count of Aversa" and a vassal of Salerno.
Guaimar set out to take possession of his new principality immediately. On 15 August, he conquered Rocca Vandra and gave it to the abbey of Monte Cassino. Meanwhile, the Normans of Aversa pacified the valley of the Sangro. After Pandulf fled to Constantinople, Guaimar turned his attention to Amalfi. In April 1039, in support of the deposed and blinded Manso II, Guaimar forced the abdication and exile of John II and his mother, Maria, a sister of Pandulf. Guaimar installed himself as duke. Then in July, he conquered Sorrento, which had been conquered by Pandulf in 1034. He gave it to his brother Guy with the title of duke. He also received the homage of the Duke of Naples, John V, who had brought the request for mediation to Constantinople in 1037.
In the north, he brought Comino, Aquino, Traetto (May 1039), Venafro (October 1040), Pontecorvo, and Sora under his rule. In June 1040, he took Gaeta, which had been conquered by Pandulf in 1032. After October 1041, Guaimar ceases to appear in the acts of Gaeta and it seems he was replaced by a popular usurper related to the old dynasty, Leo. By December 1042, however, Gaeta was in the hands of Rainulf, holding it from Guaimar.
In 1044, he and the Iron Arm began to take Calabria and built a large castle at Squillace. In his later years, he had trouble retaining his possessions in the face of the Holy Roman Emperor and the Normans. Rainulf Drengot, who still held Aversa, originally from the Duke of Naples, died in 1045 and his county passed, against all protestation from Guaimar, to his nephew Asclettin. Later in that same year, Guaimar opposed the succession of Asclettin's cousin Rainulf Trincanocte, but again was overridden. These quarrels led the once-loyal Aversa to return its allegiance to Pandulf, lately returned from exile in Constantinople. War with Pandulf continued from 1042 to 1047. Guaimar secured his own position, however, by recognising William's brother Drogo shortly after William's death in 1046 and by giving him his sister Gaitelgrima in marriage.
In 1048, Pandulf, once again prince of Capua, was at war with Guaimar. On the death in that year of Rainulf II of Aversa (Rainulf Trincanocte), his succeeding son Herman, an infant, required a regent. The first appointment, Bellebouche, was a failure. Richard Drengot, a cousin of Herman's, was then in a Melfitan prison for making war on Drogo. Guaimar soon procured his release and personally brought him to Aversa, where he was installed as regent, and later as count in his own right. Thus, Guaimar recaptured the allegiance of Aversa.
Guy fled to the Normans and soon the four conspirators were besieged in Salerno by a large Norman force and Guy's Sorrentine army. The assassins' families soon fell into their enemies' hands and they negotiated their release by releasing Gisulf, Guaimar's son and heir, to Guy. Guy accepted their surrender soon after, promising not to harm them. The Normans, however, who maintained they were not bound by Guy's oath, massacred the four brothers and thirty-six others, one for each stab wound found in Guaimar's body. Thus the Normans showed their loyalty to Guaimar even after his death.
Guaimar's legacy includes his dominion, either by conquest or otherwise, over Salerno, Amalfi, Gaeta, Naples, Sorrento, Apulia, Calabria, and Capua at one time or another. He was the last great Lombard prince of the south, but perhaps he is best known for his character, which the Lord Norwich sums up nicely: "...without once breaking a promise or betraying a trust. Up to the day he died his honour and good faith had never once been called in question.
In 1037, Guaimar had made his eldest son John co-prince as John IV, but John died in 1039. Guaimar was succeeded by his second son Gisulf II (co-prince since 1042), whom the Normans put under their protection. His third son was Landulf, Lord of Policastro. His fourth son Guy was an ally of Robert Guiscard. His fifth son was John, Abbot of Curte. His youngest son, Guaimar, co-ruled with his brother Gisulf.
Guaimar's eldest daughter was Sichelgaita, who married Robert Guiscard. His younger daughter was Gaitelgrima, whom he married to Drogo after William's death. She brought with her a large dowry. She married twice more: to Robert, Count of Monte Sant'Angelo, and to a Count Alfred. Humphrey, Drogo's brother and successor, is said to have married a daughter of Guaimar's, often assumed to be the widow of his brother, but this is impossible. Also, Jordan I of Capua is said to have married a woman named "Gatteclina", a sister of Sichelgaita.