Unlike his father, whose secondary status in the family had prevented him from making daijin, Uchimaro rose to udaijin, and led the cabinet in Heizei's reign and the beginning of Saga's.
On the ascension of Emperor Kanmu in 781, Uchimaro was promoted from shō roku-i no jō (正六位上) to ju go-i no ge (従五位下).
This came shortly after Uchimaro's first wife, Kudara no Nagatsugu, who had since become a court lady, bore the emperor a prince (Yoshimine no Yasuyo [ja] (良岑安世)).
However, in the next few years, four high-ranking cabinet officials, including udaijin Fujiwara no Tsuginawa and dainagon Ki no Kosami, died or retired, and in 798, Uchimaro was promoted to ju san-mi (従三位) and chūnagon.
When Prince Miwa (神王) died a month later, Uchimaro took over as head of the cabinet with a promotion to shō san-mi (正三位) and udaijin.
Uchimaro commissioned statues of the Four Heavenly Kings and a version of Kannon and let his son Fuyutsugu offer them to Kōfuku-ji.