Toukichirou Kinoshita (later known as Hideyoshi Toyotomi)
My girlfriend at the time (later became my wife)
この場合、次のような言い方ができますよ。
ーToukichirou Kinoshita (later known as Hideyoshi Toyotomi)
「木下藤吉郎(後の豊臣秀吉)
ーMy girlfriend at the time (later became my wife)
「当時の彼女(後の妻」」
ここの「後のXX」は later known as XX(後にXXと知られるようになる)や later became XX(後にXXになる)を使って表現すると良いでしょう。
ご参考まで!