The man was seated at a desk in a windowless basement office. Two aides had just left the room. I was alone with the new US senator from Illinois. It was 5 January 2005.

One of his feet was propped up on the desk. A government computer had been dumped on the floor, cables strewn. Fluorescent lights flickered overhead. The desk held only a few objects, including a notepad, a book (Robert Caro’s Master of the Senate: The Years of Lyndon Johnson) and a mouse pad. The senator was reading a document, paying me no mind.

Click. I made a nice candid frame of this scene, as any photojournalist would do. I was using a quiet camera. I didn’t want to disturb him. Click. I paused. Click again. I waited for a gesture, something, anything. He never once looked up at me – just went about his business. So I went about my business, too. I captured a few more frames and walked out. I had what I needed.

Obama: An Intimate Portrait by Pete Souza review – a people’s man at work, rest and play Read more

I had met Barack Obama just the day before, when he had been sworn in as a 43-year-old senator. It was a day of pomp and circumstance. He had his family in tow: a stylish wife and two young daughters. I saw right away how much those girls meant to him. He doted on them throughout the day, making sure they were comfortable. Outside the Capitol, Malia, then six, had twirled a little dance just for him. Later, inside the Capitol, he got kisses from Sasha, then three. All the while, it was as if he didn’t even notice there was this photographer with him, capturing those moments throughout the day.

How lucky for me that I was the Washington photographer for the Chicago Tribune, the young senator’s adoptive hometown newspaper. That day was the first day documenting his first year in the Senate. Jeff Zeleny, the political reporter for the paper, and I were preparing a series of major stories throughout the year, tracking how he adjusted to life in Washington. The plan was for me to spend as much time as I could with him between other assignments.

Facebook Twitter Pinterest Obama plays with daughters Sasha and Malia in the snow at the White House, 6 Feb 2010. Photograph: Pete Souza/The White House

After only 24 hours, I had already begun thinking a crazy thought. The senator was not from a famous or political family. His father was from Kenya, his mother from Kansas. His name was Barack Hussein Obama. But he had a gift. What if? I thought to myself. Could I be observing the future US president? I imagined how that photograph of him seated at his stale bureaucratic desk in his drab government office might compare with a future one of him sitting at the Resolute Desk in the Oval Office.

Thus began my professional relationship with Barack Obama. I shadowed him extensively during his first year in the Senate. We got to know each other and understand how we each worked. In 2005, I accompanied him on a congressional delegation trip to Russia, Ukraine and Azerbaijan. I remember trying to make a telling photograph of him walking around Red Square unrecognised, just in case that scene could never be repeated as he became better known. In 2006, I documented him on a family trip to Kenya, the country where his father was born. Thousands of people gathered to try to catch a glimpse of someone who at the time was still a lowly freshman senator.

Facebook Twitter Pinterest Obama helping his wife Michelle off the stage during the state dinner for François Hollande, 11 Feb 2014. Photograph: Pete Souza/The White House

I covered the start of his presidential campaign in 2007. During that summer, however, I resigned from the Tribune to start a new life as a photojournalism professor at Ohio University. But I did continue to photograph Obama and eventually published a book, The Rise of Barack Obama.

His incoming press secretary, Robert Gibbs, called me on a Sunday night in early January 2009 to offer me the job as chief official White House photographer. I remember telling Gibbs that to do the job, I needed to have access. Access to everything. All the important stuff, the moments when decisions are made. Even if it was classified. He said: “Of course, the president-elect gets it.” That was all I needed to hear.

On paper, the job is to visually document the president for history. But what, and how much, you photograph depends on each individual photographer. I thought I knew what I was in for. I had worked at the White House in my 20s as a “junior” official photographer during the Reagan administration. The person I wanted to emulate was Yoichi Okamoto, President Lyndon Johnson’s chief photographer. Previous presidential photographers had taken a lot of ceremonial pictures and few candid ones. Okamoto pushed the bar and photographed seemingly everything Johnson did.

Facebook Twitter Pinterest Obama bending over so the son of a White House staff member can pat his head, 8 May 2009. Photograph: Pete Souza/The White House

Starting on day one – 20 January 2009 – I was determined to take Okamoto’s approach. It sounds simple. But everything can change the moment a president sets foot in the Oval Office.

I had a plan for how to make it work for both of us. I would explain to him and everyone around him why I needed to be in every meeting, every day: it was my job to capture real moments for history. The highs and lows, the texture of each day, the things we didn’t even know would be important until later on. But, to paraphrase a line from Lin-Manuel Miranda’s Hamilton: “You gotta be in the room where it happens.” If you’re not, you’re not going to capture any moment, let alone a historic one.

My job was to be the observer, not the participant. Easy, right? But it was damn hard. Physically. Mentally. Spiritually. It is a job meant for someone with some experience, and a lot of youth and energy. Ideally someone in their mid-30s, maybe early-40s, tops. I started the job when I was 54.

Someone – I think it was Andy Card, White House chief of staff under George W Bush – once described working at the White House as trying to take a sip of water from a fire hose that never shuts off. That’s a pretty good analogy. Just when you think you have a break, boom, you realise that documenting the president for history is an all-consuming, 24/7, always-on-call, no-vacation, no-sick-days, BlackBerry-always-vibrating kind of job.

Facebook Twitter Pinterest Obama greets custodian Lawrence Lipscomb in the Eisenhower Executive Office Building, 3 Dec 2009. Photograph: Pete Souza/The White House

Obama and I shared a lot of time together. It was 10 to 12 hours a day, five days a week (and sometimes six or seven). I photographed every meeting, every day, every place he went to. The Oval Office, the Situation Room, the Roosevelt Room. Nearly 1.5m miles on Air Force One. All 50 states; more than 60 countries. Just shy of 2m photographs over eight years.

Along the way, we became friends. How could you not when you share so much of life together? We played countless card games on long flights overseas. He called me Pete or later the Azorean, when he found out my ancestry, and I called him Mr President or, more often, Potus. He poked fun at my age, my bald spot, the reptiles I kept at home. And I saw what delighted him, what wore on him, what made him mad and what brought him peace. Did I ever get on his nerves? Certainly. Did he get on mine? One learns to always say no.

Now, I think back to that scene from January 2005 in the basement office on his second day as a US senator. So much has changed since then. But in the 12 years I’ve known him, his character has not changed. Deep down, his core is the same. He tells his daughters: “Be kind and be useful.” And that tells you a lot about him. As a man. A father. A husband. And yes, as a president of the United States.

Obama: An Intimate Portrait – the Historic Presidency in Photographs is out on 7 November (£30, Allen Lane)