Hirosue was born the third son of Harutaka NIJO; The lord Yoshisue SAIONJI adopted him from the Nijo family, because the lord's sons successively died at an early age; Consequently Hirosue became heir to the Saionji family.
二条治孝の三男として生まれたが、西園寺賞季の男子たちが相次いで早世し、相続人がいなくなっていたため、この寛季が西園寺家に養子に入って相続することとなった。