"I assumed the ratings would be far higher than they would be normally," he said of his decision to go forward with the announcement as the nation's focus was diverted to the massive storm.
The pardon has drawn criticism from civil rights groups and even some Republicans, who have lamented the move sparing the controversial Arizona lawman from a jail term.
But Trump was unrepentant Monday, insisting Arpaio's hardline tactics were justified.
"He's done a great job for the people of Arizona. He's very strong on borders, very strong on illegal immigration," Trump said at an afternoon news conference with his Finnish counterpart. "He is loved in Arizona. I thought he was treated unbelievably unfairly."
Arpaio was convicted of criminal contempt last month for ignoring a court order in a racial profiling case. He's been accused for years of violating basic human rights in his prisons, but Trump insisted on Monday he was no worse than Americans who'd been granted clemency by previous presidents.
"Sheriff Joe is a great veteran of the military, great law enforcement person," he said, going on to list controversial clemency decisions made by past presidents, including President Bill Clinton's decision to pardon Marc Rich, the husband of a top donor, and President Barack Obama's decision to commute to sentence of Chelsea Manning, who leaked classified information to Wikileaks.
Trump even suggested Obama pardoned Manning -- "horrible thing that he did, commuted the sentence and perhaps pardoned," he said -- though Obama only reduced Manning's prison sentence, and did not offer her a full pardon.
Trump's pardon of Arpaio was his first use of presidential clemency powers, which are sweeping and don't require consultation among other administration agencies.
The Justice Department said on Friday that Trump did not consult with lawyers there before announcing the decision to pardon Arpaio.
The decision drew backlash even from some members of Trump's own party, including Arizona's two Republican senators and House Speaker Paul Ryan.
But Trump was defiant on Monday.
"Sheriff Joe is a patriot. Sheriff Joe loves our country. Sheriff Joe protected our borders. And Sheriff Joe was very unfairly treated by the Obama administration," Trump said. "I stand by my pardon."