Liverpool attacker Luis Suarez, the player of the tournament, was also on target as Uruguay claimed a first continental crown since 1995 and broke their all-time Copa title tie of 14 which they held with Argentina.
For Forlan, it was an emotional triumph as he followed his father and grand-father into the record books as a Copa America winner.
"This really means a lot for me because my grand-father won it, my father won it and now I have also done it,'' said the former Manchester United striker, who took his international tally to 31 goals, a record he now holds with Hector Scarone.
"Three generations have won this trophy. the name of Forlan will stay in history."
Suarez opened the scoring in the 12th minute when his drive took a deflection off Dario Veron and left goalkeeper Justo villar wrong-footed.
Atletico Madrid star Forlan added the second three minutes from the interval when Egidio Arevalo cleverly intercepted the ball to release the veteran attacker who confidently buried a left-footed strike.
It was Forlan's first goal in 13 matches as Uruguay, who knocked out hosts Argentina in the quarter-finals, were in cruise control at River Plate''s famous Estadio Monumental.
Suarez added: "We really wanted this, we are a united group and we deserved it. We had to fight like crazy to win this and we've achieved something great." Captain Diego Lugano said the win was a triumph for the whole of the country.