A second half penalty from Leigh Griffiths was enough to seal the points for the hosts, bringing an end to a four-match unbeaten run for St Mirren who remain second bottom of the SPL.
"I was very disappointed with the manner in which we went about our business," Lennon said. "A great deal of credit must go to Hibs for them coming here and stopping us playing.
"It's very uncharacteristic that you get a group of players and 11 of them have an off day at the same time. We'll just put this down as a bad day at the office.
"We now have seven games left and we must now take each one as it comes and make sure we try and hit a run and then achieve our ambition of making it into that top six. It's going to be difficult, but not beyond our means."
Griffiths was awarded his spot-kick opportunity after Saints skipper Jim Goodwin was penalised by referee Willie Collum for a challenge on winger Matt Done, who reacted theatrically on contact.
On the hotly-contested decision, Lennon added: "I didn't see it at the time, the fourth official was standing in my way.
"But when I saw the ball come in I didn't think there was any apparent danger. We seem to be getting conflicting stories from the officials. Until I see it again, that's as much as I'll say on it."