In 1599, Katsunari returned to Tokugawa service, as he reconciled with his father and Ieyasu Tokugawa. At this time, Katsunari was given him domain worth of approximately 10,000 koku worth. On April 22 of the same year, Katsunari's younger sister, Kanahime (later known as Seijoin), became Ieyasu's adopted daughter and married to Katō Kiyomasa. In 1600, Katsunari participated in Eastern army during
Sekigahara Campaign and being entrusted to lead the army of
Mino Province to the campaign of
Aizu. However, on July 18, Tadashige was murdered by
Kaganoi Shigemochi, an ally of Mitsunari. Then Katsunari were appointed to inherit the position of his father as the head of Mizuno clan. Katsunari, who became the head of the Mizuno family, returned to
Kariya castle due to the cancellation of the Aizu conquest and went to the Battle of Sekigahara. /Hyūga Masamune, Formerly in possession of
Ishida Mitsunari; It was seized by Mizuno Katsunari In Ogaki castle As the armed confrontations between the Eastern army and the Western army started, Katsunari was tasked in command of Sone castle under the command of
Ii clan and
Honda clan under Ii Naomasa and
Honda Tadakatsu to suppress the enemy in Ogaki. However, as there are detachments of Western army approaching
Gifu Castle, were tasked to take their forces to Gifu, while leaving the command of Sone castle to the Katsunari while he also ordered to strengthen the castle defense. On September 13,
Shimazu Yoshihiro led his soldiers to besiege Sone Castle and bombarding Sone castle with their artilleries. Facing this situation, Naomasa and Tadakatsu implored Katsunari to repulse the Shimazu forces. In response, Katsunari goes out with sallying force with his younger brother, Mizuno Tadatane, to defend Sone Castle. Katsunari ordered his artilleries to return fire at the turret of the Shimazu artilleries, and he then led his army storming the Shimazu position and manage to overcome the
Shimazu clan's army, causing Yoshihiro to retreat and abandon the siege against Sone castle. After he beat the Shimazu forces, Katsunari returned to Ieyasu to ask permission to participate on the Sekigahara main battle which planned in the next day. Ieyasu instead ordered Katsunari to guard Sone castle and keep an eyes on Ogaki castle which located nearby and being controlled by Western army loyal to Mitsunari. Katsunari then continues by burning the wall and outer citadels of the Ogaki castle. As Katsunari burned the outer citadels of Ogaki castle, the Western army nearby that area, including the Shimazu clan who has just beaten by Katsunari's army, decided to abort their plan to go into as they thought the Ogaki castle could not be saved. Then Yoshihiro decided to retreat into
Ise Province. After the victory of Tokugawa army in the main
battle of Sekigahara, Katsunari immediately persuade the defender of Ogaki castle,
Akizuki Tanenaga, to surrender, which immediately accepted. Katsunari entered Ogaki castle and immediately wrote a letter to Ii Naomasa to give pardon to Tanenaga, which accepted by Ieyasu. he also seized the Hyūga Masamune blade belongs to Ishida Mitsunari, This caused the extinction of the Kaganoi clan. In December 1608, It was reported that Katsunari met with
Miyamoto Musashi. Musashi taught Katsunari the secret techniques of his swordstyle.
Campaign against Toyotomi Hideyori In 1614, Katsunari participated during the Tokugawa forces
Siege of Osaka against
Toyotomi Hideyori. It was believed that
Miyamoto Musashi fought on this battle under his banner, and become one of Katsunari's son mounted bodyguard. Furthermore, Musashi's adopted son,
Miyamoto Mikinosuke, also served under Katsunari. According to the
Sekisui zatsuwa, Mikinosuke was also Musashi's paternal relative through Nakagawa Shimanosuke, who was also a vassal of Katsunari. The attendance of Musashi in this conflict under the Mizuno clan also attested by contemporary Oba clan record documents. A document titled
Munekyu-sama Godeko (a collection of Katsunari's words and actions after his retirement) also mentions Mikinosuke. On December 20, peace was established between Tokugawa and Toyotomi. As the reclamation of the moat became a condition for peace, Katsunari also became involved in the reclamation of the moat at Aoyaguchi, northeast of
Osaka Castle. After that, he served as the guard at the Kuromonguchi (Sakuramon) gate of Osaka Castle, and returned to Kariya in February. In 1615, conflict against Hideyori broke again. Katsunari participated this summer phase of Osaka siege, he was appointed as the first commander in chief of the Yamatoguchi area (Yamato area army). Katsunari was given strict order to not initiate the assault towards the castle. Then he mobilize his troops from Kyoto to Nara city, where he met an enemy forces led by Ōno Harufusa who were on their march to attack Koriyama castle at Nara. However, Harufusa army immediately disengage and retreat after they saw Katsunari forces. After he arrived in Nara, Katsunari rendezvoused his army with
Honda Tadamasa Matsudaira Tadaaki,
Date Masamune,
Matsudaira Tadateru, and others. On May 4, Katsunari was called by Shogun
Tokugawa Hidetada and went to
Fushimi, where he received an award of 50 gold pieces for his efforts in preventing the enemy from attack city Nara. Later, Katsunari taking his army of 3,800 soldiers south through the night, and on the May 5, He set out with Hori Naoyori,
Matsukura Shigemasa, Bessho Magojiro, Okuda Tadatsugu, Niwa Ujinobu, Nakayama Terumori, Murase Shigeharu, and others, and arrived at Kokubu at 4 p.m. to camp. On the May 6, Katsunari fought against
Gotō Mototsugu, his former colleague when still serving
Kuroda clan, in the
Battle of Dōmyōji. Katsunari then further pursuing the enemies and catch up against Usuda Kanesao, who were killed by one of Katsunari's lieutenant. As he pursue further, he met up with the rear guard of the Toyotomi army, led by
Sanada Yukimura,
Mōri Katsunaga,
Akashi Takenori, and Ōno Harunaga. At this point, Katsunari asked Date Masamune repeatedly to keep pursuing the enemies, which rejected by Masamune as he stated lacks of ammunition and the casualties mounted so far. In the final engagement against the Toyotomi forces on the
Battle of Tennōji, the Echizen Matsudaira forces were defeated by Akashi Nobutsuna and fled to Katsunari's forces. When Yukimura lost his momentum of suicidal charge in an attempt to kill Ieyasu, Katsunari attacked Sanada's troop with about 600 men from the west of Shoai-in Temple. Sanada's corps, attacked from three sides, was finally destroyed. Otani Yoshiharu, a lieutenant of Yukimura, was killed by Katsunari's troops during the mop up operation. Katsunari planned his war banner at Sakura gate. == After death of Ieyasu ==