In doing so, he made history becoming the first Latino to address the convention in the keynote role. His speech evoked a young Barack Obama when he was the convention keynoter in 2004.
He made a personal and a political speech. Unlike New Jersey Gov. Chris Christie in the Republican keynote speech, Castro perfectly balanced his own personal story and at the same time made the argument to re-elect the president.
Could Castro run for president one day and possibly win?
It's possible. He has a few things in common with Obama.
He went to Harvard Law School. He was raised by a single mother.
There also are differences.
Castro, 37, was elected mayor in 2009 and said he plans to run for re-election. Before that he was on the San Antonio City Council.
Castro is likable and would appeal to the growing Hispanic demographic in the United States.
He probably needs more experience, to win a statewide office, such as Texas governor, or a Senate seat before he could run for president.
One thing is certain. His role and the speeches of many other Latino politicians and celebrities like actress Eva Longoria, who will speak Thursday night, show the importance and growing power of the Latino vote.
Obama won 70 percent of the Latino vote in 2008 and he needs at least that and more. But it won't be as easy this time as he hasn't been able to pass immigration reform since he has been blocked by Republicans and some Democrats too.
He did, however, change policy to allow undocumented youth, also known as Dreamers, to apply for work permits and defer deportation.
Romney will likely get less Latino votes than former president George W. Bush and Sen. John McCain. Bush won 40 percent of the Latino votes in 2004 and McCain won 30 percent in 2008. I don't expect Mitt Romney to do any better than McCain and likely worse.
But the biggest risk for President Obama is that some Latinos will sit out this election because they are disappointed in him. The key will be Latino voter turnout.
The Obama Administration is counting on speeches from Castro and others to mobilize the Latino vote.
It could make all the difference especially in toss-up states like Florida, Colorado and Nevada.
Castro's pro-education message helped. And so did his mention of Obama's effort to defer deportation of DREAM Act youth.
"And because he knows that we don't have an ounce of talent to waste, the president took action to lift the shadow of deportation from a generation of young, law-abiding immigrants called dreamers," Castro said in the televised speech.
He spoke of his grandmother, an orphan who left Mexico and moved to Texas. She had a fourth grade education and worked as a maid and cook.
She probably would have thought it extraordinary that one grandson would be mayor of San Antonio and another on his way to the U.S. Congress, Castro said.
"My family's story isn't special. What's special is the America that makes our story possible. Ours is a nation like no other, a place where great journeys can be made in a single generation. No matter who you are or where you come from, the path is always forward," Castro said.
He also thanked his mother.
"My mom fought hard for civil rights so that instead of a mop I could hold this microphone," Castro said.
Maybe one day Castro will run for president. He certainly held the national stage Tuesday night and President Obama is hoping he will help him win over Latino voters.