His first managerial job was with the Giants, starting in 1947. He managed them for three years, winning the final "one league system" championship in 1949. Despite that championship, however, he was replaced as Giants manager by his long time rival, fellow Shikoku native Shigeru Mizuhara.
Mihara took over the newly created Nishitetsu Lions in 1951 (the Lions were formed as a merger of the Nishitetsu Clippers and Nishi-Nippon Pirates) and led them to much success on the field. He won Pacific League pennants with them in 1954, 1956, 1957 and 1958. The Lions lost to the Dragons in the 1954 Nippon Series, but beat Mizuhara's Giants in three straight Nippon Series from 1956 to 1958.
In 1960, Mihara became the manager of the Taiyo Whales and led them to their first ever Nippon Series championship (over the Daimai Orions). The Whales had finished last the previous six seasons, but under Mihara, the Whales finished 4.5 games ahead the Giants in Mizuhara's final season as their manager.
At this point, he had six first place finishes and four Nippon Series championships in 13 years of managing. Although he would manage another 13 years, this was the high point of his managerial career.
He remained manager of the Whales until 1967, finishing second twice to the Tigers in 1962 and 1964. He moved to the Kintetsu Buffaloes from 1968 to 1970, finishing second to the Hankyu Braves in 1969. His final three seasons managing were with the Yakult Atoms from 1970 to 1973, finishing in the second division all three years. (He was in fact the final manager of the Yakult Atoms - Yakult returned to the name Swallows in 1974.)
He is the all time leader in games managed in Japan with 3248, 40 or so more than Katsuya Nomura. He is second all time in managerial wins with 1687, more than 100 more than Mizuhara (although in almost 500 more games). I think he's first all time in managerial losses with 1453 (Japan Baseball Daily's numbers are inconsistent between the listing for his managerial record and the list of all time managerial records). He was elected to the Hall Of Fame in 1983, one year before he passed away.
I don't have any cards of Mihara from when he was an active manager, but he's shown up with some regularity in the recent OB sets, including the 2004 Giants 70th Anniversary set, both the Buffaloes 2009 Memorial set and this year's "Legend Of The Bs" sets and a number of the recent sets for the Lions. For whatever reason, he did not show up in the 2009 Yakult 40th Anniversary set. Here's some sample cards:
|1978 NST #74|
|2008 BBM Lions Classic #01|
|2006 BBM Nostalgic Baseball #041|
|2009 BBM Kintetsu Memorial #20|
It's not clear from the uniform if the picture on the NST card was from his playing days with the Giants (1936-38) or his managerial days (1947-49).
I think that the Lions were highlighting the fact that this would be the Mihara's centennial during the Lions Classic games last summer.
Sources for this post include the always useful Japan Baseball Daily's Data Warehouse along with Gary Engel and Rob Fitts' "Japanese Baseball Superstars".
UPDATE: I don't know why I didn't think to look when I was writing this, but Baseball Reference's Bullpen has an excellent biography of Mihara.