In December 1179, as a result of TAIRA no Kiyomori's coup (Coup of the Third Year of Jisho), Tadatsuna was appointed as Saemon Shojo (Jr. Lieutenant of the Left Division of Outer Palace Guards) through a Jimoku (ceremony for appointing officials) and also received the imperial decree to become a kebiishi (a police and judicial chief).
治承3年（1179年）11月、平清盛のクーデター（治承三年のクーデター）に伴う除目により左衛門少尉に任ぜられ、検非違使宣旨も受けた。