Messing first rose to prominence as the goalkeeper for New York University's football team, for whom he played during his freshman and sophomore years (1968-69). In his second season, he was named second team All-America. He transferred to Harvard University for his junior and senior years (1970-71), again earning second team All-America honors in 1971.
He joined the US national team at the 1971 Pan American games in Colombia and again for the 1972 Summer Olympics in Munich. His only appearance was a 7-0 loss to Germany in the United States' final group stage match of the Olympics.
Messing graduated from Harvard in 1973 and signed for the New York Cosmos of the NASL as a backup keeper. He appeared for them only nine times before being transferred to the Boston Minutemen in 1975. Reportedly, his transfer was partially due to the Cosmos management's unhappiness with his having posed for a nude photo shoot that appeared in Viva magazine. Messing later joked that the photo shoot gave the Cosmos more media exposure than they had received up to that point.
Messing rejoined the Cosmos in 1976 and was the starting keeper for the side that won the 1977 Soccer Bowl, alongside Pelé, Franz Beckenbauer, and Giorgio Chinaglia. He left New York after the 1977 season and played for several teams from 1978 to 1987, spending six years (1979-1984) with the Rochester Lancers/New York Arrows.
He is currently a broadcaster and analyst covering MLS and European matches.