After serving as an officer of the Osaka-jo Castle and the Nijo-jo Castle, he became one of the Guards of Osaka-jo Castle on August 14, 1648 to be provided 10,000 koku, in addition to 7,000 koku which he already had, in Settsu Province such as in Arima County and Kawabe County (Hyogo Prefecture), and Nose County and Teshima County and, as a result of that, he became a daimyo (Japanese feudal lord) of 17,000 koku to be counted among lords and founded the Iino Domain.
その後、大坂城や二条城の在番を務め、慶安元年（1648年）6月26日、7000石を領していた正貞は大坂定番となって摂津国有馬郡・川辺郡(兵庫県)・能勢郡・豊島郡(摂津国)などにおいて1万石を加増されたことから、1万7000石の大名として諸侯に列し、ここに飯野藩を立藩した。