Leroy (Satchel) Paige, one of the folk heroes of baseball's old Negro leagues who became a rookie pitcher in the major leagues at the age of 42, died yesterday at his home in Kansas City, Mo.

He was believed to be 75 years old when he died after a long siege of heart trouble and emphysema. But his exact age was one of the mysteries in the legend that accompanied him into the big leagues in 1948 with the Cleveland Indians, and it was still a mystery when he pitched his final three innings for the Kansas City A's in 1965 when he was admitting to 59.

That was the end of what he laughingly called ''my 100-year career in baseball.'' And, by then, he was celebrated for his homespun wit as well as for his fastball and stamina, and most especially for his admonition: ''Don't look back. Something might be gaining on you.''

View From Rocking Chair

By then, he was viewing the world from a rocking chair in the Kansas City bullpen, a tall, thin man with a thin mustache who had lived one of the phenomenal careers in sports: 22 years as a barnstorming pitcher in the era before black players were admitted to the big leagues, then five seasons with three clubs in the American League, including the World Series of 1948.

In the barnstorming days, he pitched perhaps 2,500 games, completed 55 no-hitters and performed before crowds estimated at 10 million persons in the United States, the Caribbean and Central America. He once started 29 games in one month in Bismarck, N.D., and he said later that he won 104 of the 105 games he pitched in 1934.

By the time Jackie Robinson signed with the Brooklyn Dodgers in 1947 as the first black player in the majors, Mr. Paige was past 40. But Bill Veeck, the impresario of the Cleveland club, signed him to a contract the following summer, and he promptly drew crowds of 72,000 in his first game and 78,000 in his third game.

Five Full Seasons

His career in the big leagues was spread over 18 years but, because he retired twice during that span, it totaled only five full seasons with these statistics: 28 victories, 31 defeats, 476 innings, 290 strikeouts and an earned-run average of 3.29. But by then, he already occupied a special rank as a showman and country philosopher who advised people to ''avoid running at all times,'' and who once reflected:

''There never was a man on earth who pitched as much as me. But the more I pitched, the stronger my arm would get.'' Despite the uncertainty about his age, there was general agreement that he was the oldest player ever to appear in a major league game when he pitched three innings against the Boston Red Sox on Sept. 25, 1965. And, two years later, clearly enjoying his role as an athletic phenomenon, he wrote an autobiography with the title: ''Maybe I'll Pitch Forever.''

Leroy Robert Paige was born in Mobile, Ala., the son of John and Lula Page. The family name became ''Paige,'' he remembered, because ''my folks later stuck in the 'i' to make themselves sound more hightoned.''

He teased people about the date of his birth, saying that the certificate had been placed between the pages of a Bible that was eaten by the family's goat. But later he did not argue with evidence that he had been born on July 7, 1906.

Origin of His Nickname

He was specific, though, about his nickname. He got it as a 7-year old while hustling baggage at the railroad depot in Mobile after he had invented a contraption for carrying more bags.

''I rigged up ropes around my shoulders and my waist, and I carried a satchel in each hand and one under each arm,'' he said. ''I carried so many satchels that all you could see were satchels. You couldn't see no Leroy Paige.''

He took up pitching during four years spent at the Alabama Reform School for Boys, and became exceptional. In 1924, he presented himself to Candy Jim Taylor, the manager of the Mobile Tigers, a black semiprofessional team, and fired 10 fastballs past the manager in an audition. He had a job, and soon a career.

For the next two decades, he traveled around the hemisphere wit

h black teams, pitching across the seasons and the borders of countries. He also pitched in exhibition games against white major league stars. Once he outpitched Dizzy Dean, 1-0. Another time, he struck out Rogers Hornsby five times in one game. Joe DiMaggio called him ''the best I've ever faced, and the fastest.''