It was only United's third Cup final victory - in this the 100th year of the competition - with the decisive moment arriving shortly before the break.
The 32-year-old's left-foot strike was also his first goal of the season for a club who have been struggling during the regular MLS season.
United have won just three times in 30 MLS games and, while they have no chance of completing a cup and league double, this triumph has qualified the team for a crack at the North American Champions League.
Meanwhile, Real Salt Lake had gone into the match with a real chance of securing the double.
They are currently second in the MLS with 51 points, sharing most wins in the league so far this season with Seattle and New York.
As for Leicester-born Neal, he made his professional debut for Stoke City while still a teenager in 2000 and went on to play for Shrewsbury, Carlisle, Notts County and Preston before heading Stateside in 2011.
By coincidence, he had a trial with Salt Lake last year but was not offered a contract and he duly signed for DC United in March 2012.
As for the match, Salt Lake were the dominant side and came close to equalising on 59 minutes when a long-range strike by Sebastian Velasquez crashed against the bar.
United keeper Bill Hamid also made a brace of excellent saves, but it was Neal who made the most significant contribution by firing home a loose ball from inside the box that sneaked inside the right-hand post.