In 1566, he became a retainer of Nobunaga when Nobunaga attacked Mino Province. He worked under Hashiba Hideyoshi (Toyotomi Hideyoshi) from then on, and was involved in various battles such as the Battle of Nagashino, battle against Ishiyama Hongan-ji and battle against Saika party at Kii Province. In 1582, he served Hideyoshi after Nobunaga died, and was active in several battles such as the Battle of Yamazaki, Battle of Shizugatake, siege of Odawara, and Battle of Bunroku. He was given 171,800 koku at Sanuki Province because of his credit. In Hideyoshi's last years, Chikamasa, Nakamura Kazuuji and Horio Tadaharu were appointed to three chu-rō and participated in the Toyotomi administration.
In 1600 at the Battle of Sekigahara, Chikamasa took part in Ishida Mitsunari's force while his son, Ikoma Kazumasa took part in Tokugawa Ieyasu's force. Chikamasa was at Sanuki, and send off his retainers as his substitute to attack against Tanabe castle at Tango Province. He chose the method to keep Ikoma clan going, even if which force was defeated.
Because Kazumasa took part in Ieyasu's force, Ieyasu let Chikamasa rule his domain before the battle continuously after the battle. Chikamasa took his responsibility for his taking part in Mitsunari's force, and transferred the headship of the family to Kazumasa. He became a priest and withdrew to Mount Koya.
He was permitted to back in Sanuki Province before long. In 1603, he died in Takamatsu castle.