The 35-year-old left back darted into the box and met Eoin Doyle's left-wing cross with a beautifully-timed volley into the far corner from ten yards, for his first goal for his new club, who he joined in the summer.
Humphreys said: "No one likes to pick up a left back when he's going on a long run like that. It was a delightful ball in to me from Eoin.
"It's been an amazing few hours for me."
Chesterfield boss Paul Cook sent his goalkeeping coach Eric Nixon out to speak to the media after what was a sixth clean sheet in eight games from the Spireites.
Nixon said: "It's fantastic for us to get another clean sheet, but most of all to get the three points.
"Congratulations to Ritchie and his wife for their new arrival. He took the goal well and we felt we could have had a few more as well.
"We played some fantastic football throughout the game, and with respect to Oxford I thought we were comfortable today.
"It's been a great start to the season with these six clean sheets in eight, but we're keeping our feet firmly on the ground and are not getting carried away with ourselves."
Chesterfield's cause was helped by Oxford losing their skipper, Andy Whing, to a red card after 67 minutes for a dangerous tackle on Doyle.
Nixon said: "It was a bit of a rash tackle, but we've all been there."
Chesterfield might have added further goals in the game with U's keeper Ryan Clarke stopping a near-post effort from Tendayi Darikwa in the first half and turning aside Jay O'Shea's drive late on.
Deane Smalley drove wide from a good position for Oxford, and also headed against a post late on, but once Whing was dismissed, Oxford were always up against it as Paul Cook's team maintained their four-point lead at the top of League Two, this time from Fleetwood, who have taken over in second place.