There are certainly plenty of examples from the second half of the 20th century onwards of non-judges being appointed to the Court:

• Associate Justice Elena Kagan, appointed by President Obama in 2010, was Solicitor General of the United States prior to her appointment in 2009, and has never served as a judge (in 1999, President Clinton nominated her for a federal judgeship, but the Republican-controlled Senate stalled her nomination.)
• Associate Justice (later Chief Justice) William Rehnquist, appointed by President Nixon in 1971, was an Assistant Attorney General in the Justice Department prior to his appointment, and also never served as a judge prior to joining the Court.
• Associate Justice Lewis Powell Jr., also appointed by Nixon in 1971, spent his entire career prior to joining the Court in private practice. (His nomination was in large part the result of his prominence in conservative legal circles; he was the author of the Powell Memorandum, an influential 1971 document that's seen as a roadmap for the modern American conservative movement.)
• Associate Justice Thurgood Marshall, appointed by President Johnson in 1965, was serving as Solicitor General at the time of his appointment; he had previously served on the US Court of Appeals, but only for 4 years.
• Associate Justice Abe Fortas, also appointed by Johnson in 1965, spent his entire pre-Court career in private practice — including serving as Johnson's personal lawyer. He never served as a judge prior to the Court.
• Associate Justice Arthur Goldberg, appointed by President Kennedy in 1962, was serving as Secretary of Labor at the time of his appointment. Before that, he was a prominent labor lawyer.
• Associate Justice Byron White, also appointed by Kennedy in 1962, was Deputy Attorney General before joining the Court. Before that, he spent his career in private practice.

Obviously, we're dealing with some very small numbers here; only 114 people have ever served on the Supreme Court. But of the 22 justices appointed since 1960, more than a quarter had never served as judges.

Certainly, previous judicial service tends to be seen as a major advantage for SCOTUS nominees; a lack of judicial experience was a key reason that George W. Bush's 2005 attempt to put his White House Deputy Chief of Staff Harriet Miers on the Court failed.

Among other things, one reason that federal judges tend to be favoured for SCOTUS is that they've already had to go through Senate confirmation (unless they were a recess appointment); the thinking is commonly that if they've passed muster once, they should be able to do it again.

Given the massive increase in political polarisation in Congress since the 1980s — along with the rise of the Christian right and the shift in conservative legal thought led by the Federalist Society, which have both made the Supreme Court an ever more active ideological battlefield — tactical questions about what a credible nominee looks like, how they'll be perceived by the media and public, and whether the Senate will confirm them, have become key factors in presidents' decisions about SCOTUS appointments.

This isn't to suggest that there was some lost golden age of meritocratic appointments — look at Court nominations at any point in American history, and you'll find examples of justices appointed on the basis of ideological compatibility with the administration, personal relationships and political expediency. From Storm Center: The Supreme Court in American Politics, David O'Brien's influential book on the Court (this is from 2008):

The Senate tends to vote for nominees who are well qualified and ideologically compatible with their constituents. However, senators may turn against nominees who are viewed as too ideologically extreme... In any event, the ultimate verdict on candidates for the Court turns less on merit than on political visibility, support and circumstance.

[...]

Thirty nominees to the Court have fallen prey to partisan politics, either because of opposition to the nominee's political views and due to the mobilization of interest groups against a nominee, or because the Senate wanted to deny lame-duck presidents appointment to the high court. [...] Only two nominees — George Williams in 1873 and G. Harrold Carswell in 1970 — suffered defeat because of mediocre professional qualifications.

Have a look at Henry Abraham's history of SCOTUS nominations, Justices, Presidents and Senators: A History of US Supreme Court Nominations from Washington to Bush II for more information. Jeffrey Toobin has written two very good books on the modern Court — The Nine and The Oath — that are illuminating on how the rise of the new conservatism since the 1960s and the culture wars since the 1980s have changed the Court's role in American life today.

Edit: typo

I don’t have full knowledge of your question and my hope is that someone with more knowledge of Supreme Court history can go to into further detail. However, I’ll try and answer your question.

The constitution doesn’t specifically specify that a Supreme Court Justice must have a law degree, a required age, experience or citizenship.

Hugo Black and Fred Vinson both had a bachelor of laws. Earl Warren had a Juris Doctor.

As for Hugo Black, FDR wanted a committed and ardent supporter of the New Deal. FDR disliked the conservative justices of the Supreme Court ruling against the New Deal, so he tried to appoint more justices (the constitution does not specify an amount). That failed. Then in 1937, 5 years after he had been elected he finally got to appoint a justice. He chose Hugo Black because he was an ardent new dealer and wanted him to uphold his policies. I don’t believe FDR really cared a lot about Legal Experience.

Fred Vinson was on the DC Circuit court of appeals from December 15, 1937 – May 28, 1943.

Earl Warren had been Californian AG and a DA.

They all had some legal knowledge and experience however certainly not as much as Supreme Court justices do now.

With regards to non-judicial backgrounds, whilst some law schools that admitted blacks and women existed, there were very few and a President certainly wouldn’t appoint a woman or a non-white with explicit racism being ubiquitous in politics at the time. This would mean that there were less qualified candidates if there was less education. A lot of men also fought in WW1.

It also doesn’t necessarily mean they’ll be a bad judge. William Douglas, Earl Warren and Louis Brandeis were excellent justices in my view even if they lacked experience.