Baseball was notorious for having the Negro leagues exist, and the story of Jackie Robinson breaking the colour barrier in 1947 is something that has been covered in depth. However what policies were done to keep black players out of the NFL and NBA (and it's predecessors) until they were both integrated?
(I've deliberately left hockey out of this, as I'm familiar with it; however I'm happy to expand on that for anyone who would request it)
“If I have to integrate heaven, I don’t want to go." That's a quote from Woody Strode, who along with Kenny Washington helped break the color barrier for the NFL in 1946.
You may know Strode now more from his post-football acting career. You can watch him here fighting Kirk Douglas in Spartacus. You can also see him here in the opening of Sergio Leone's Once Upon a Time in the West and as the lead in John Ford's Sergeant Rutledge, a Western about a black U.S. calvary soldier falsely accused of rape and murder of a white woman and her father.
This is how many know Strode. But Strode and Kenny Washington were, as Sports Illustrated put it, "the Jackie Robinsons of the NFL"...in 1946. This was a year before Jackie Robinson played in the MLB (1947) but Robinson had already signed with the Dodgers. He played with the minor league Montreal Royals the season before.
How did the NFL break the color barrier first? It wasn't progressive policies. It wasn't a codified policy at all. To answer your question about what policies were done, the phrase used is "gentlemen's agreement." Which is to say, the NFL owners simply didn't hire any black players from 1933-1946. It's more complicated than simply not hiring.
First, understand the NFL wasn't the behemoth in the US during the early 20th century as it is now. From 1920 - 1933, no one had Super Bowl parties. Or even NFL Championship parties. The NFL was made up of small-market teams (like the Green Bay Packers in Green Bay as opposed to the larger Milwaukee) and the champion was whoever had the most amount of regular season wins (like the English Premier League today). Most players had other jobs in the off-season such as "Red" Grange who worked in ice delivery.
Before there was a NFL commissioner, there was the NFL President and between 1921-1939 that was Joe Carr. For Carr, the point of the NFL was to become more organized in the same way as Major League Baseball. Carr took strides to make football run like Major League Baseball with the goal of long-term league stability and to build a fanbase for professional football teams. For the most part, the US viewed professional football with skepticism or scorn. At best, the NFL seemed like a "gimmick." Football was a college sport to most Americans, not a professional sport.
Carr helped negotiate the contract for "Red" Grange that amplified awareness of the NFL, for instance. He also understood that small market teams could not be the future. It was unsustainable in the long run to have the Akron Pros or the Columbus Panhandles. They needed teams in the big cities just like Major League Baseball. Carr recruited wealthy owners from these cities like the Mara Family who started the New York "Football" Giants. These owners could pour money into these teams, connect with other wealthy investors and patrons, and afford to pay them to functionally operate during lean times...like the Great Depression.
All of this is to say that by 1933, the NFL was in a relatively precarious situation for brand awareness, legitimacy, and finances. The period from 1920 - 1933 was a fluid time when the NFL could falter at any given time. It was also an integrated league. Fritz Pollard, a Brown University collegiate who signed with the Akron Pros and played in 1920. Also on the Akron Pros for a time: actor Paul Robeson. Joe Lillard even played for the Chicago Cardinals as late as 1932. Plus, you had Jim Thorpe and the Oorang Indians - a team of Indigenous football players.
But in 1933, in the Great Depression, a new owner came into the NFL - George Preston Marshall. A West Virginian, Marshall became the owner of the Boston Braves which became the Boston Redskins which became the Washington Redskins (and is now the Washington Football Team).
It's hard to argue against an idea that Marshall was racist and a segregationist. Greg Howard in The Big Book of Black Quarterbacks calls him a "visionary and a bigot." To add perspective: the Washington Redskins didn't integrate until 1962!
Howard credits Marshall for things such as Divisions and the NFL Championship. Marshall (and Carr) understood that to project stability, they had to have regularly scheduled games, a special championship that provided bonuses, and halftime pageantry - things an poor league or team wouldn't do.
He also points to Marshall as the main reason for the color line. It was never codified as a policy. But Marshall refused to hire black players and in a saturated labor market with a lot of white players (the Great Depression), others followed. Marshall did this for his racist, paternalist, and segregationist beliefs but it impacted how other owners (who had their own racial biases) operated. If Marshall wasn't hiring, none of them would or it might look less "legitimate" to white audiences. For the owners and the NFL during this time, to project financial strength to a mostly white patronage they had to not look so desperate that they had to resort to hiring Black and non-white players. The fact that Marshall could field an all-white team meant the other teams would look less viable and "legitimate" for investment.
But this was never a spoken policy. Thomas G. Smith in Showdown: JFK and Integration of the Washington Redskins even quotes former owners who DENY that there was a color line.
"For myself and for most of the owners," Art Rooney of the Pittsburgh Steelers explained to me decades later, "I can say there never was any racial bias." George Halas of the Chicago Bears declared to sportswriter Myron Cope in 1970 that there had been no unwritten exclusionary agreement, "in no way, shape, or form." Tex Schramm of the Los Angeles Rams did not recall a gentleman's agreement: "You just didn't do it—it wasn't the thing that was done." Tim Mara of the Giants also denied that minorities had been blackballed.
The closest confession to a "gentlemen's agreement" came from George Preston Marshall. From The Atlantic:
In a 1942 interview, Marshall argued that if black players were allowed to participate, Smith writes, "white players, especially those from the South, would go to extremes to physically disable them," so they were kept off the field in their own best interests
From 1933-1946 there were no black players. And in the 1940s, that saturated labor market from the 1930s changed as many players joined to serve in WW2. Chicago Bears' owners George Halas would even sign Bronko Nagurski out of retirement and from his wrestling career instead of hiring a black football player in 1943. Halas would later argue that "no great black players were in the colleges then."
But that's untrue. Jackie Robinson, before he became a Brooklyn Dodger, was a football player with UCLA. Who else was on his team? Woody Strode and Kenny Washington. The UCLA team was historic - an integrated backfield that went undefeated for the 1939 season. Even Halas scouted players on this team for the NFL. In 1939, the NFL moved to a 20 round draft system.
Not one of the UCLA players was signed by a NFL team.
Until 1946.
The Cleveland Rams moved from Cleveland to Los Angeles in 1946. Many of the football teams didn't own their stadiums. They leased their stadiums from professional baseball teams (the Chicago Bears played at Wrigley Field, for instance) or from municipal stadiums. The Rams moved to LA and were to play at the tax-payer funded LA Coliseum.
Halley Harding, a former Negro League baseball player and sportswriter for the Los Angeles Tribune, put pressure on sports teams for failing to integrate among other Black media writers. When the Rams came to LA and agreed to play at the LA Coliseum, Harding spoke at the Coliseum Commission's hearing, stating that "since black tax dollars helped build and maintain the Coliseum, none of its tenants should be allowed to discriminate." Now, could the Rams fight this? Possibly. But they gave in after meeting other black LA sportswriters. They signed former UCLA and hometown athletes Woody Strode and Kenny Washington.
They were old within football years and faced violent defenders on the gridiron (hence the beginning quote), but Washington still led the league in 1946 for yards per carry.
Afterwards, professional football teams began to sign black players. Even George Preston Marshall in 1962 because Robert Kennedy threatened to push Marshall out of DC Stadium if he refused to integrate.