The unconditional withdrawal -- four months later than a deadline agreed with the Taliban last year -- comes despite a deadlock in peace talks between the insurgents and the Afghan government, which threatens to leave a power vacuum that could plunge the country deeper into violence.
Blinken met Afghan President Ashraf Ghani as well as senior US officials in Kabul and briefed them on President Joe Biden's announcement Wednesday that he was ending "the forever war", which began in response to the 2001 September 11 attacks.
"I wanted to demonstrate with my visit the ongoing commitment of the United States to the Islamic Republic and the people of Afghanistan," Blinken said after meeting Ghani.
"The partnership is changing, but the partnership is enduring."
In a separate meeting with Abdullah Abdullah, who is leading the Afghan government side of the peace process, Blinken said it was the start of "a new chapter that we're writing together".
The Pentagon has around 2,500 troops in Afghanistan from a high of over 100,000. Thousands more serve as part of a 9,600-strong NATO force, which will withdraw at the same time.
Prior to meeting Ghani, Blinken visited the US embassy in Kabul and told an audience made up mostly of soldiers: "What you and your predecessors did over the last 20 years is really extraordinary."
The US pullout has caused consternation in Afghanistan, where citizens live in fear of daily bombings and targeted assassinations by an emboldened Taliban.
"It could cost us losing everything we worked and fought together for in the past 20 years and could risk overall security in Afghanistan," said Metra Mehran, a Kabul-based women's rights activist.
For the US, however, after 20 years and more than 2,400 deaths, enough is enough.
"We cannot continue the cycle of extending or expanding our military presence in Afghanistan hoping to create the ideal conditions for our withdrawal, expecting a different result," Biden said on Wednesday.
The delayed withdrawal -- even by just over four months -- has angered the Taliban, who have threatened to resume hostilities against US forces.
"If the agreement is breached and foreign forces fail to exit our country on the specified date, problems will certainly be compounded and those who failed to comply with the agreement will be held liable," spokesman Zabihullah Mujahid said on Twitter on Wednesday.
The US military has not engaged in direct combat with the insurgents since last year's agreement, but it provides vital air support to Afghan forces engaged in daily warfare.
After a phone call with Biden Wednesday, President Ghani said Afghan forces are "fully capable" of controlling the country.
Fighting continues unabated on the ground despite months of talks in Qatar between the Taliban and Afghan government negotiators.
Turkey announced this week that it would host a separate international peace conference on Afghanistan from April 24, but the Taliban have said they will not attend.
The US wants the conference to get the Afghan government and Taliban to agree to some form of unity interim administration and map a future for the country by consensus.
Ghani is seeking a brokered ceasefire followed by elections, while the Taliban -- without being specific -- want Afghanistan to become an Emirate again, ruled under Islamic law by religious elders.
