The March total is a 24% increase from the same month a year earlier, when 169,000 migrants were picked up at the border, the start of a rise in migration that left thousands unaccompanied children stuck in crowded border patrol stations for days while they awaited placement in overwhelmed government-run shelters, Reuters reported.
Biden, a Democrat who took office in January 2021, pledged to reverse many of the hardline immigration policies of his Republican predecessor, former President Donald Trump, but has struggled both operationally and politically with high numbers of attempted crossings.
Republicans, who hope to gain control of the US Congress in Nov. 8 midterm elections, say Biden's rollback of Trump-era policies has encouraged more illegal immigration.
Biden officials have cautioned that migration could rise further after US health officials said they will end a pandemic-era border order by May 23. The order, known as Title 42, allows asylum seekers and other migrants to be rapidly expelled to Mexico to prevent the spread of COVID-19.
While more than half of the migrants encountered at the US-Mexico border in recent months have been from the traditional sending countries of Mexico, Guatemala, Honduras and El Salvador, migrants have increasingly been arriving from more far-flung places, including Ukraine and Russia.
US officials are preparing for as many as 18,000 migrant encounters per day in the coming weeks, but are also readying for smaller increases.
The 210,000 migrants arrested in March, a figure made public in a court filing on Friday night, is the highest monthly total on record since February 2000, according to US Customs and Border Protection statistics dating back to 2000.
Another 11,000 migrants attempted to enter at a legal crossing along the southwest border in March without a valid visa or permission, the court filing said.
Roughly half of the migrants encountered in March were expelled under the Title 42 order, the court filing said.