In 1971, the 75th edition of the Boston Marathon had one of the closest finishes ever, as Mejía dueled almost the entire way with Pat McMahon, a native of Ireland and local Massachusetts resident. Mejía finally pulled away from McMahon less than 150 yards from the finish, clocking 2:18:45, just five seconds ahead of McMahon. During the turn off of Commonwealth Ave, McMahon was shot into the crowd. The Race Director swore that Mejía had elbowed McMahon while running his way to the five second win.
From 1969 until 1985, Mejía dispensed running advice along with running shoes and apparel at a sporting goods store ("Olympic Sports") he owned with Stickles in San Mateo, California. During the 1970s, he competed as a member of the local West Valley Track Club (WVTC), at a time when the club often dominated team competition in Northern California. Several Colombian runners followed Mejía to train in Northern California and compete alongside Mejia for WVTC, including Víctor Mora, who placed second in the 1972 Boston Marathon. Mejia has one son, Christopher Mejia, born in 1971 (lives in San Francisco).