Singh held on to win the Volvo Masters for his second victory of the season, shooting a 1-over 72 to edge Harrington, Sergio Garcia and Luke Donald by one shot at 2-under-par 282.
Harrington, meanwhile, pushed past Paul Casey for the yearlong Order of Merit title as the European Tour's leading money winner -- parlaying his almost implausible 30th career second-place finish in Europe into his first Harry Vardon Trophy.
'It's great to finish second, and yet win,' said Harrington, the second Irishman to win the Order of Merit after Christy O'Connor went back-to-back in 1961-62.
'It's something I've always wanted to do. As a goal at the start of the year, I always want to win the Order of Merit.'
Harrington closed with a 2-under 69, tying his best score of the weekend after making three birdies on the back nine and two crucial up-and-down pars at the 17th and 18th holes.
He then sat for more than an hour in the clubhouse, watching as the scenario that would award him the Order of Merit unfolded.
'It's very hard when you can't do anything about it,' Harrington admitted. 'I never like watching golf that I'm involved in because I don't like wishing people bad, but I obviously don't want to be sitting there watching people make birdies, either.
'So, I found it very difficult sitting there for the last hour and watching the scoring.'
Garcia was tied with Singh on the back nine, but he knocked his approach into the sand at 18 and couldn't get up-and-down for par. The closing bogey gave him a 1-over 72 and his third runner-up finish at this event.
Behind Garcia, Singh was doing his best to make sure the Spaniard's score wouldn't matter anyway.
After sitting over his ball for a long time in the 17th fairway, Singh knocked his second shot within 12 feet, then two-putted for a birdie.
Admitting afterward that he didn't look at the scoreboard until the last hole, Singh believed he might be trailing when he finally lined up for his fairway shot at the par-five 17th.
'I was thinking somebody was at three or 4 under,' Singh said. 'I thought I needed to eagle  or birdie it at least to have a chance on the last hole.'
At 3 under par himself, Singh only needed to make a bogey at the par-4 18th for the win -- which he did after pushing his tee shot into the right rough at Valderrama's tough closing hole.
'The last hole, I'm happy the way it finished,' said Singh.
Singh, the first Indian golfer to qualify for the European Tour when he won his card in 1997, also won the Volvo China Open in April. Sunday's victory was his first on European soil.
'I was calm and a little tentative, too, on a few holes,' Singh said of his final round. 'But I just told myself to focus on the process, and I think that helped me.'
Donald had a 2-under 69 to end alongside Harrington and Garcia at 1-under 283. Among the remaining pack, three other Order of Merit hopefuls saw their chances dashed.
David Howell, once hopeful to become the first wire-to-wire Order of Merit champion before a shoulder injury and Casey got in his way, closed with a 71 and tied for fifth place with Niclas Fasth (71) at even-par 284.
Robert Karlsson -- who entered the tournament ranked fourth behind Casey, Harrington and Howell -- stumbled to a 4-over 75 and fell from a tie for eighth place into a tie for 21st at 4-over 288.
As for Casey, he never could recover from a flu-induced 76 in the opening round. Sunday, he finished with his best score of the tournament, a 2-under 69 that moved him into a tie for 21st place alongside Karlsson.
In the end, Harrington won the Order of Merit with just around 35,000 euros more than Casey.
'I feel sorry for Paul Casey,' Harrington sighed. 'Obviously, everything went against him.'