As he tweeted through his last day in office, one of his final engagements was launching the Compassionate Belfast Charter at City Hall.
The charter aims to promote "an all-inclusive and peaceful society" in the city.
Those in attendance signed the charter, recited the prayer for Belfast, enjoyed musical performances representing traditional Irish, Scottish and African compositions, and enjoyed poetry read by Sinead Morrissey.
The ceremony concluded with the Lord Mayor making a special presentation of gifts to his chaplains from all faith communities, as acknowledgment of the work they did throughout the year.
During his time as the First Citizen, the Sinn Féin Balmoral councillor has handed over the freedom of the city to Van Morrison, and got on his bike for the Giro d'Italia Big Start.
He has become known for embracing social media and generally being game for anything, including dying his hair pink for the Giro and even dressing as a superhero for a special UTV April Fool's.
I think the people of the city don’t mind pomp and ceremony but what they really want is someone who could put a smile on the face of Belfast and I tried that.
Máirtín Ó Muilleoir
He said on his last day: "It's been a manic, fanatic year but I do have the best job in Belfast.
"For me the standout moments were personal and public.
"When the city came together fearless Oscar Knox died, we lit up the city hall in yellow and teal, the colours of Team Oscar, lanterns went out above the sky of Belfast, that was a very special moment of unity in the city.
"Presenting the freedom of the city to Van Morrison who wrote about moving the city from the dark end of the street, to the bright side of the road, was a very powerful moment of healing for the city."
He also said he treasured the day that a local Presbyterian congregation held their Bible class at city hall and said prayers for him.
"Those are moments that may only happen to you once in a lifetime, they are moments which I will treasure and cherish," he added.
"For me this job is all about building the peace, building the reconciliation between our people and I am grateful for the welcome I received across the city and that people made space for me, especially unionist people, to meet them half way and build the city together."
At the start of his term in office, the Sinn Féin member pledged to represent all the people of Belfast and champion diversity.
Councillor Ó Muilleoir said that an attack on him last August by loyalist protestors during a civic opening at Woodvale Park never deterred him from his work.
He continued: "Those who attacked me in the park do not represent the great people of Woodvale or Shankill.
"The vast majority of those people believe in co-existence, mutual respect, a peaceful future for Belfast, which is no place for violence.
"I was back many times in loyalist areas and the message to the people of the city was if you can represent everything that's good about Belfast.
"If you can be positive about the city, you will find that echoed in every community
"My job is to shine a light on that, to let people know what a great city we have and what great people there is."
SDLP north Belfast councillor Nichola Mallon takes over as the new Lord Mayor of Belfast, while Alliance councillor Maire Hendron is the new Deputy Lord Mayor.