Lockman played a supporting role in one of the most famous ninth-inning comebacks in baseball history. On October 3, 1951, Lockman scored the tying run, just ahead of Bobby Thomson, on Thomson's home run that gave the New York Giants the National League championship - baseball's "Shot Heard 'Round the World." Lockman's one-out double against the Brooklyn Dodgers had scored Alvin Dark with the Giants' first run of the inning, and made the score 4-2, Brooklyn. His hit knocked Dodger pitcher Don Newcombe out of the game, and, on the play, Giant baserunner Don Mueller broke his ankle sliding into third base. While Mueller was being carried off the field to be replaced by a pinch runner, Dodger manager Chuck Dressen called on relief pitcher Ralph Branca, whose second pitch was hit by Thomson into the Polo Grounds' lower left field stands for a game-winning, three-run homer.
Lockman was a first baseman and outfielder who batted left-handed and threw right-handed. He signed with the Giants as a 17-year-old during World War II and came to New York from the minor leagues in the midseason of 1945, just past his 19th birthday. He batted .341 in limited duty that season, but would hit over .300 only once more during a 15-year major league playing career. He was a regular in the Giant lineup from 1948 through June 1956, when he was traded to the St. Louis Cardinals. His exile was brief, however, as the Redbirds sent him back to the Giants after the end of the 1956 season. He was a member of the Giants' last New York team, and their first San Francisco outfit, when the club moved West in 1958. He finished his career in 1959-60 with the Baltimore Orioles and Cincinnati Reds.
Appearing in 1,666 games, Lockman had a .279 career batting average with 114 home runs. In the 1951 World Series, won in six games by the New York Yankees, Lockman hit .240 with a home run. Three years later, he batted only .111 in the 1954 Fall Classic, but the Giants swept the Cleveland Indians to win the world championship.
Lockman's coaching career began immediately after his playing days ended, as he joined the Reds' staff in 1960 under skipper Fred Hutchinson. In 1961, when his old mate, Dark, became manager of the Giants, Lockman became his third base coach, serving through 1964. Lockman then joined the Chicago Cubs as a minor league manager, coach, and, then, Director of Player Development. In July 1972, he succeeded his old mentor, Leo Durocher, as Cubs' manager and the revitalized Cubbies won 39 of 65 games to improve two places in the standing. But losing marks in 1973 and into 1974 cost Lockman his job; he was relieved of his duties July 24, 1974 and moved back into the Chicago front office. Lockman later was a player development official and special assignment scout for the Montreal Expos and Florida Marlins.
Lockman finished with a career major league managing record of 157-162 (.492).