It was awarded the title in May 2022 as part of the Queen's Platinum Jubilee celebrations.
Camilla, the Queen Consort, was unable to attend after a positive Covid test.
Anti-monarchy protesters who joined the crowds were outnumbered by royal supporters chanting "God save the King".
A reception was held at the Milton Keynes' Church of Christ the Cornerstone, where the King met representatives from local charities and businesses.
During a speech at the church, the King described Milton Keynes as "something special in its own right, that we can celebrate with its inhabitants at this moment in its history".
After the ceremony at the church, the King visited the Milton Keynes (MK) Food Bank to see the support it provides to communities across the city.
One of the themes of the King's first Christmas address was the cost of living crisis, and the broadcast featured footage of a food bank.
Louisa Hobbs, operations manager of MK Food Bank, which supports more than 5,000 families across the city, showed King Charles around and said his interest was clearly "genuine" after he explicitly requested to visit a food bank in Milton Keynes.
Milton Keynes first applied for city status in 2000, then in 2002 for the Golden Jubilee, and again in 2012 for the Diamond Jubilee.
It was eventually awarded the title in May 2022 during the Platinum Jubilee events.
In January 1967, plans for a new town were approved - and soon a quiet Buckinghamshire village became the vast development of Milton Keynes.
The then housing minister Anthony Greenwood granted permission to transform an 8,850-hectare area of villages and farmland into a town for 250,000 people.
It was a grand vision - part of the third and final phase of the government's plans to relocate populations from London and other cities that suffered immense bomb damage during World War Two.
Some five decades on, Milton Keynes has a population of about 280,000 and has become home to more than 10,000 businesses.
It is used as a model for new towns across the world.
The new city is home to the distance-learning institution, the Open University, which was established by Royal Charter in 1969 and is the largest university in the UK in terms of students enrolled.
The Milton Keynes University Hospital NHS Foundation Trust established the nation's first independent medical school, and Milton Keynes was the first place to introduce kerbside recycling in the UK.
Today, 140 languages are spoken in Milton Keynes' schools including those of every Commonwealth nation, and 30 religions are observed, the council said.