In the battle of Sekigahara in 1600, Yoshimasa was in the West squad along with his father Hidemasa, and joined the siege of Tanabe-jo Castle in Tango Province, where Fujitaka HOSOKAWA (also known as Yusai HOSOKAWA) secured.
慶長5年（1600年）の関ヶ原の戦いのとき、父・秀政と共に西軍に属し、細川藤孝（幽斎）が守る丹後国田辺城(丹後国)攻撃に参加した。