"We thought we'd have the concert and the government would change things," he said before taking the stage at the almost-annual concert Saturday.
When Mellencamp, Willie Nelson and Neil Young founded the charity in 1985, they were mostly concerned with saving family farms, which were going out of business fast in a time when land values were falling and interest rates were soaring.
The mission has changed over the years. With that farm crisis passed, the organization is mostly concerned now with connecting farmers and consumers. Musicians talked as much Saturday about getting kids to eat fresh food as they did about keeping farms commercially viable.
"We started out trying to save the family farm," Nelson said at a pre-concert news conference in which the rockers sat among farmers. "Now, it looks like the family farm is going to save us."
Over the years, Farm Aid has raised some $29 million, which it uses to help farmers in crisis and support organizations with missions like setting up farmers markets and encouraging farmers to go organic.
The emphasis at this year's event — the farthest Farm Aid has gone into the heavily populated Northeast — was on teaching urbanites about where food comes from.
Tina Johnson, who founded a grocery cooperative to open next year in the struggling community of Chester, Pennsylvania, just south of Philadelphia, said kids in that area do not know where their food comes from.
"You ask a child where milk comes from, they'll tell you 'the store.' They won't tell you 'a cow,"' she said.
Farm Aid's message may have been secondary to the music. Some fans donned faux Willie Nelson-style braids, and many wore the T-shirts of their favorite artists.
Concertgoer Hillary Aderman stopped by a table run by the Land Stewardship Project to sign a petition urging Congress to pass legislation next year to help small farms.
"I came for the bands, but I support the cause," Aderman said.
Others downed organic beer, pork sandwiches from a Missouri family's hog farm, soy milk and $9 organic hamburgers.
"I'm tired of those hot dogs and hamburgers," said Toni Matlosz, gesturing to the regular amphitheater fare.