Norman Mineta was put into an internment camp as a teenager, and became a mayor in 1971 and in the House of Representatives in 1975. Of course that is decades later, but still, only a generation later it seemed to have completely passed at least in this specific example.
When did the prejudice and discrimination generally end at a notable scale?
There's a great book about this topic called "America's Geisha Ally: Reimagining the Japense Enemy" by Naoko Shibusawa. It goes into great detail about the United States government entering into the "reverse course" following WWII. What this basically means is that during and directly after the war, the general consensus of the government and populace of the United States was that Japan was going to pay dearly for its aggressive war. However, because of the looming threat of communism in the far east, and the capitalist framework that Japan had in place (infrastructure, skilled and disciplined workforce, industry, etc.) these plans were scrapped in favor of a program that would promote Japanese economic strength and stability. The United States went through great lengths to basically 'retrain' its populace from seeing the Japanese as a hated natural enemy to seeing them as effeminate and weak and in need of pity. Great book if you get a chance to check it out!
How quickly did prejudice towards Japanese-Americans by the general American population end after WW2?
Any academic who studies race and racism in the US will tell you that prejudice and discrimination has not ended against Japanese people in the US. It may not be as widespread as it was decades earlier, but many US citizens hold stereotypes about the Japanese. Some of this discrimination is related to WW2. Prejudice and stereotypes are passed down in families and across generations. Some discrimination is related to portrayal of Asians and Japanese in the US media.
Japanese in the US today often face discrimination, just as other non-white people do. The degree of this varies across US states, cities and rural locations. There is less discrimination in places that are more diverse, such as parts of California. Strong anti-Japanese feelings are also more common in older people in the US. It is also true that anti-Japanese discrimination has decreased in the last two decades.
Most experts would not say discrimination against Japanese in the US has ended, but may agree that attitudes towards Japanese people have changed over time (and become less negative). Note that the perspectives on historical and contemporary racism in the US vary greatly between these three groups: academic experts on race relations; whites; and non-whites that experience discrimination (including Japanese Americans and immigrants).
From Racial Microaggressions and the Asian American, 2007, Columbia University
The next three quotes are from this source:
Despite the belief that Asian Americans have somehow “made it” in our society and are “immune” to racism, widespread prejudice and discrimination continue to take a toll on their standard of living, self-esteem, and psychological well being (Wong & Halgin, 2006). Indeed, the study of Asians in America is the study of widespread prejudice and discrimination leveled at this group.
Evidence of prejudice and discrimination today:
most of the racial microagressions that occurred came from peers, neighbors, friends or authority figures. It disturbed them that personal or respected acquaintances could make such insensitive or hurtful remarks. What bothered them most, however, was their occasional tendency to “make excuses” for friends by rationalizing away their biases and by denying their own racial reality.
Many whites and non-whites in the US are in denial and dismiss the idea that discrimination exists and is widespread in the US. Many non-whites have a different view based on their personal experiences of discrimination.
it is important for social scientists and the general public to possess a realistic picture of Asian Americans and to understand the many overt and covert manifestations of racism directed at them.
Given all of this, including the existence of frequent, subtle discrimination today against Japanese Americans, it is still true that the view of Japanese by the general public is very different today than in the decades after WW2. Discrimination and prejudice in the US changes and evolves over time.
Based on your example of Norman Mineta, your question may really be:
When did the strongest forms of overt racism and prejudice against Japanese and Japanese Americans end in the US? You note this Japanese American Congressman as evidence that this prejudice has ended.
Additional Source:
Race, Rights, and the Asian American Experience, Angelo N Ancheta, Rutgers University Press, 2006.
racial discrimination, both subtle and overt, persists in many sectors of American life, and Asian Americans still suffer from treatment both as "perpetual foreigners" and as a "model minority" group.
For anyone who wants to learn more about race relations in the US (from an academic perspective) I highly recommend taking a college course on racism, or using a syllabus from such a course as a guide to reading materials that serve as an introduction to the subject.