Father abandons mother and son when he's six months old, mother is unable to raise son alone so she sends him to his grandparent's farm (which, in this case, was in Slater, Missouri), son is in and out of trouble, moves back in with mom, goes back to farm, joins the circus (no, I'm not kidding), reunites with his mother in the big city (Los Angeles), joins a gang, commits petty crime, gets sent to a boys reform school in Chico, graduates and eventually joins the Marines, receives an honorable discharge 3 years later, uses the money from his G.I. Bill to study acting, begins to race motorcycles on the weekend to make ends meet while playing small roles in play productions, works his way into bigger parts, gets launched into limelight after acting alongside Frank Sinatra in Never So Few, earns and executes an exceptional performance in his first starring role in The Magnificent Seven, the legacy begins, and boom goes the dynamite.
Note: McQueen used to demand bulk goods (jeans, electric razors, etc.) from the studios he did work for. It was later discovered that he would donate these items to the boys at the reform school he attended as a kid.