Spearmon, whose lane violation late in the Beijing Olympic final cost him a bronze medal, surged ahead with 50 meters remaining and took the trials title in a wind-aided 19.82 seconds, a 2.3m/sec tailwind boosting him to the finish.
"That's something that has definitely been on my mind since 2008," Spearmon said. "It's hard enough to make one team. If I didn't make the team this year, that would have been on my mind for the rest of my life.
"This is a chance at redemption. I don't take my second chances lightly. I'm going to go out there and leave it all on the track."
Maurice Mitchell was second in 20.14 with Isiah Young third in 20.16 to join Spearmon in qualifying for the London Olympics, where they will face Jamaican superstars Yohan Blake and Usain Bolt, the reigning world and Olympic champion.
"As far as London, I don't really have times set," Spearmon said. "Everyone expects Jamaica to be 1-2. We could go out there and get first, second and third. You never know what to expect."
Shawn Crawford, the 2004 Olympic champion and 2008 Olympic runner-up in the 200, failed in his bid to reach London, finishing seventh in 20.37.
Former world champion Bershawn Jackson, third at the 2008 Olympics, also will miss London after being edged for third place in the men's 400m hurdles final.
"I'm disappointed. Not making the team is really tough," Jackson said. "We live for the Olympics. It's heartbreaking. It's probably the toughest race of my career that I messed up in."
Michael Tinsley, who was eliminated in the first round of the 2008 trials, was a shock 400m hurdles winner in 48.33, booking a surprise trip to his first Olympics at age 28.
"I'm just thankful to be in these guys' presence. It was an honor to come out on top," Tinsley said.
Angelo Taylor, the 2000 and 2008 Olympic champion, was leading comfortably when his left foot struck the penultimate hurdle, and that jostled the hurdle in Jackson's adjacent lane.
"I saw Angelo hit hurdle nine and when I saw that I just went into overdrive," Tinsley said.
Jackson went off stride and it cost him dearly at the finish.
"Angelo Taylor hit my hurdle and threw off my stride pattern," Jackson said. "When that hurdle got hit, it threw my rhythm off."
Taylor, 33, was thrown off stride but cleared the last hurdle and finished second in 48.57 to give himself a chance at a third Olympic title.
"I was really going for it when I clipped hurdle nine and that took the life out of me," Taylor said. "I'm thankful I was able to regain my composure and get over hurdle 10 and finish strong. It meant so much to make my fourth Olympics."
Former world champion Kerron Clement, a 2008 Olympic runner-up, dove across the finish line for third in 48.89, edging Jackson by .05 for a London berth.
"I knew I had to dive. I just dug deep and got third," Clement said.
World champion Lashinda Demus won the women's 400m hurdles in 53.98, becoming the second-fastest performer in the world this year behind Russia's Irina Davydova.
Two-time world champion Brittney Reese won the women's long jump with the best leap in the world this year at 7.15m. Janay DeLoach, who jumped a wind-aided 7.15 in qualifying, took third in 7.08 to join her on the team.
Morgan Uceny, last year's Diamond League champion, won the women's 1,500 in 4:04.59 to book a trip to London, as did runner-up Shannon Rowbury in 4:05.11 and reigning world champion Jenny Simpson, who took third in 4:05.17.
Leo Manzano, a 2008 Olympic semi-finalist, won the men's 1,500 in 3:35.75 with Matthew Centrowitz, third at last year's worlds, another .09 back in second to book his trip to London.