I have recently been reading David G. Dalin's *Jewish Justices of the Supreme Court: From Brandeis to Kagan.* One thing he discusses is people other than Brandeis who might have become the first Jewish Justice of SCOTUS:
(1) The earliest was Judah Benjamin, who was actually offered the post by President Fillmore. He turned it down. Would he have been confirmed if he had accepted the nomination? Probably. It is true that the Senate, controlled by Democrats, was in no particular mood to confrm a Justice named by a lame-duck Whig president, and had turned down Fillmore's other choices for the vacancy. But it was widely believed that the Seante would not refuse to confirm a fellow senator (even a newly-elected one like Benjamin) and the fact that Benjamin was a Deep Southerner and a defender of slavery would help him with Democratic senators, especialy from the South. (Of course this could also hurt him with northern antislavery Whig senators; Benjamin Wade of Ohio famously called Benjamin "an Israelite with Egyptian principles.")
(2) In 1910 there was a serious lobbying effort to get President Taft to appint Louis Marshall to the Supreme Court.
https://en.wikipedia.org/wiki/Louis_Marshall Marshall was one of America's leading consitutional and corporate lawyers, a long-standing member of the judiciary committee of the New York Bar Associaton, and a partner in what was then the nation's leading Jewish law firm, Guggenheimer, Untermeyer, and Marshall. He was an influential Republican and friend of some of New York's leading Republican lawyers like Charles Evans Hughes and Elihu Root. However, Taft detested Marshall's law partner, the Democrat and anti-"money trust" crusader Samuel Untermeyer. Even though Untermeryer and Marshall differed politically, Taft would just not appoint a partner of Untermeyer's. Moreover, Taft may already have promised the Court vacancy (caused by Justice Brewer's death) to Hughes. So Marshall was probably never a real prospect.
(3) Dalin writes that "It might also be argued that had Taft wanted to appoint a Jew to the Supreme Court to attract, or retain, Jewish support, he would have chosen Mayer Sulzberger
https://en.wikipedia.org/wiki/Mayer_Sulzberger rather than Marshall. A pillar of the Republican Party in Philadelphia, Sulzberger enjoyed the personal respect of Taft, who wrote that he was 'proud to claim' Sulzberger as a friend. More significant, perhaps, Sulzberger possessed an important qualification for the appointment that Marshall lacked--prior judicial service. When Taft entered the White House in 1909, Sulzberger had served with great distinction for fourteen years on the Court of Common Pleas, Philadelphia's principal judicial tribunal, the last seven of those years as President Judge. Such arguments notwithstanding, Taft seems unlikely to have considered Sulzberger for any of the six high court appointments he during his tenure as President. According to Lloyd P. Gartner, the claimy that Taft offered a Supreme Court seat to Sulzberger, 'who politely declined on account of his age and desire to pursue Jewish scholarship,' seems unfounded."
https://books.google.com/books?id=JqwNDgAAQBAJ&pg=PA16
(4) Taft did appoint Julian Mack
https://en.wikipedia.org/wiki/Julian_Mack to the US Court of Appeals for the Seventh Circuit in 1910. Perhaps if Taft had been re-elected in 1912, he would have appointed Mack--like Marshall and Sulzberger, a staunch Republican--as the first Jewish justice on SCOTUS.
Can anyone think of any other possibilities?
(I have chosen to put this in the post-1900 section because of the four potential Justices I have considered, three of them would have been appointed in the twentieth century.)