In Mexicali, his tone turned a bit caustic. He averted his gaze from indigent seniors splayed out on the ground asking for money. He pointed at the irony of a church situated next door to a brothel.

“This is what you’ll find here,” he said. “Poverty, drugs and prostitution.”

If his feelings toward his American citizenship had been fraught in the past, adulthood clarified a few things. He began to wonder what it meant that he was technically American, what opportunities might be open. He started working toward an American high school equivalency diploma, though he admits his lack of English proficiency remains a significant obstacle.

“My mom told me a short time ago, ‘Look, I couldn’t give you nice things like other people have. But I gave you American citizenship, which is what so many people are fighting for,’” he said.

He recognizes that now. “But I feel Mexican, I’m not going to lie to you,” he said.

When he turned 18, seeking a foothold to life in the United States, Mr. Posada got a job at one of the nearby farms that supply a large portion of Americans’ winter vegetables. It was backbreaking work. He would awaken at 1 a.m. and cross the border with other migrant farmworkers and begin picking by 3. He saw the physical toll the work had taken on Mexican men who had done it for decades.

That was not the future he wanted, he thought. “It’s a job that kills you.”

Now, he has a job registering low-income residents of Calexico for government-funded wireless plans on cellphones once colloquially known as “Obama phones.” Because nearly everyone in Calexico speaks Spanish, Mr. Posada can do the job without language getting in the way.