The seven-time world champion was behind title-rival Max Verstappen and team-mate Valtteri Bottas all afternoon at Barcelona's Circuit de Catalunya before eventually clinching top spot by 0.036secs from the Red Bull driver.
Reacting immediately, Hamilton said: "I can't believe we are at 100 and it is down to the men and women down at the factory who continue to raise the bar and just never give up.
"The support that I have, it has been a dream for me to work with them and the journey we have been on... who would have thought when we partnered we would be qualifying at 100 so I feel very humbled and grateful. I am ecstatic like it is my first."
Asked about whether a 100th pole felt special, such as for a cricketer scoring a century or a footballer netting his 100th goal, Hamilton added: "I can't compute it.
"I don't know how I am supposed to feel. It's such a big number it's hard to express how crazy and amazing it is."
Hamilton explained changes made ahead of qualifying had left him concerned as the tweaks taken with his W12 were a step in the wrong direction.
"FP3 was looking pretty good and we have been strong all weekend," he added.
"We made some changes, I was pretty anxious about the changes we were going to make for qualifying because we are always trying to make the car better but it was a bit of a gamble because you have got to keep the race in mind also.
"We made this change and I was like 'this is the wrong way' but it was my call in the end.
"I was making small tweaks here and there to try and elevate myself pace-wise. The first lap was the best I did all session and then I tried to improve on the next lap and I was a tenth and a half up but I couldn't keep it."
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